J OURNAL OF Journal of Petrology, 2017, Vol. 58, No. 1, 85–114 doi: 10.1093/petrology/egx006 P ETROLOGY Advance Access Publication Date: 15 March 2017 Original Article

Pleistocene to Growth of a Large Upper Crustal Rhyolitic Magma Reservoir beneath the Active Laguna del Maule , Central Nathan L. Andersen1*, Brad S. Singer1, Brian R. Jicha1, Brian L. Beard1, Clark M. Johnson1 and Joseph M. Licciardi2

1Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706, USA; 2Department of Earth Sciences, University of New Hampshire, Durham, NH 03824, USA

*Corresponding author. E-mail: [email protected]

Received June 17, 2016; Accepted January 26, 2017

ABSTRACT The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36S, is among the most active latest –Holocene rhyolitic centers globally and has been inflating at a rate of > 20 cm a–1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, 20 km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic pre- cursors to the LdM result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of the LdM are decoupled from the slab fluid signature, similar to several recently studied fron- tal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope com- positions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcan- ism during the early post-glacial (25–19 ka) and Holocene (c. 8–2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element com- positions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus silicic cumulates. A long-term rate of mass addition—extrapolated from surface deformation accu- mulated over the past decade—is comparable with those that have produced moderate- to large- volume -forming eruptions elsewhere.

Key words: rhyolite; Southern Volcanic Zone; magma chamber; geochronology; radiogenic isotopes

INTRODUCTION (ignimbrite) deposits are often interpreted to reflect the Large silicic volcanic systems are of great interest because structure of the pre-eruption magma reservoir (e.g. they generate caldera-forming eruptions that disperse Hildreth, 1981). The composition and ages of major and enormous quantities of ash over a vast area. accessory phases can provide records of magma accumu- Heterogeneities in the resulting pyroclastic fall and flow lation, crystallization, and mixing on both short (100–102

VC The Author 2017. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] 85 86 Journal of Petrology, 2017, Vol. 58, No. 1 year) and long (104–106 year) timescales (e.g. Vazquez & melting of the deep crust (up to 70% depending on the Reid, 2004; Charlier et al., 2005, 2008; Wark et al., 2007; magma flux and lithology of the crust), fractional crys- Costa, 2008; Reid, 2008; Reid et al., 2011; Wotzlaw et al., tallization of hydrous , and mixing of the resulting 2013, 2015; Chamberlain et al., 2014a, 2014b). differentiates and crustal melts. Shallow systems are Complementing these records are studies of smaller pre- assembled incrementally from these lower crustal ‘hot and post-caldera silicic eruptions that record the longer zones’ (Annen et al., 2006), but undergo limited chem- thermochemical context that produced the caldera- ical differentiation following shallow magma emplace- forming system, particularly when the earlier or subse- ment. Thus, the volume of eruptible magma is primarily quently erupted material is physically distinct from the a function of the magma flux to the upper crust (e.g. caldera-forming system or the caldera-collapse event pro- Glazner et al., 2004; Annen et al., 2006; Annen, 2009; duces a structural realignment of the shallow magma sys- Gelman et al., 2013). tem (Metz & Mahood, 1985, 1991; Sutton et al., 2000; The investigation of pre-caldera silicic eruptions can Charlier et al., 2005; Smith et al., 2005, 2010; Simon et al., provide clues to the physical and thermal evolution that 2007; Wilson & Charlier, 2009; Bachmann et al., 2012; sets the stage for the assembly and eruption of a volu- Barker et al., 2015). minous silicic magma reservoir. Pre-caldera eruptive re- The archetype model of voluminous silicic magma cords can be limited owing to infrequent eruptions, systems involves crystallization of to intermediate poorly resolved geochronology, burial or destruction by forerunners in the middle to upper crust, yielding an subsequent caldera-forming events (Metz & Mahood, intermediate to silicic crystal mush—an extensive 1991; Stix & Gorton, 1993; Wilson et al., 2009). crystal-rich (>60% solid) reservoir containing evolved Nevertheless, such records have proven useful in iden- interstitial melt. Crystal-poor eruptible magma bodies tifying changes in the mafic flux to the upper crust, the are assembled by progressive extraction and accumula- amalgamation of previously discrete magma reservoirs, tion of melt from these crystal-rich domains (Bachmann and placing limits on the longevity of the subsequent & Bergantz, 2004; Hildreth, 2004) or remelting of silicic caldera-forming reservoir (e.g. Metz & Mahood, 1991; cumulate during magma recharge events (Mahood, Simon et al., 2007; Bindeman et al., 2008; Wilson & 1990; Wolff et al., 2015; Evans et al., 2016). The relative Charlier, 2009; Chamberlain et al., 2014b). importance of these mechanisms varies between Understanding the recent magmatism at historically ac- caldera-forming systems as well as within zoned ignim- tive rhyolitic volcanic centers (e.g. Miller, 1985; Hildreth, brites produced during individual events (e.g. Vazquez 2004; Smith et al., 2005; Castro & Dingwell, 2009; & Reid, 2004; Charlier et al., 2005; Bindeman et al., Hildreth & Fierstein, 2012; Rawson et al., 2015) allows 2008; Wotzlaw et al., 2013, 2015; Chamberlain et al., for the interrogation of the structure of the magma res- 2014a, 2014b; Evans et al., 2016). ervoir, the petrogenesis of rhyolites, the physical and Departures from the model of progressive rhyolite thermal processes preceding the recent eruptions, and extraction have been noted at large silicic systems such their evolution through time. Such systems are poten- as Taupo Volcano and Yellowstone involving a greater tial sites of caldera-forming eruptions and, taken to- proportion of remelting of silicic forerunners and the gether, this information is valuable in evaluating the amalgamation of distinct rhyolite melts, potentially cat- possible style of future eruptions and establishing a alyzed by extensional tectonics (Smith et al., 2004, 2010; context in which to better interpret seismic, magnetotel- Charlier et al., 2005, 2008; Wilson et al., 2006; Shane luric, geodetic, and gravity observations (e.g. Singer et al., 2007, 2008; Bindeman et al., 2008; Wilson & et al., 2014). Charlier, 2009; Allan et al., 2013; Be´ gue´ et al., 2014; The rear-arc Laguna del Maule (LdM) volcanic field Storm et al., 2014). Brief repose periods following the (Fig. 1) produced two dacitic to rhyodacitic caldera- eruption of compositionally distinct pre-caldera rhyo- forming eruptions during the mid-Pleistocene. A recent lites, durations of zircon crystallization, and crystal resi- concentration of silicic volcanism has yielded at least 50 dence based on solid-state diffusion kinetics indicate rhyolitic eruptions in the last 26 kyr; thus LdM is among that the assembly of 102–103 km3 eruptible rhyolite the most frequently erupting active rhyolitic volcanic magma bodies in these systems occurred more rapidly centers globally (Hildreth et al., 2010; Fierstein et al., than predicted by models of progressive melt extraction 2012; Sruoga, 2015). This remarkable spatial and tem- (Charlier et al., 2008; Allan et al., 2013; Bindeman & poral concentration of rhyolite eruptions since the last Simakin, 2014; Wotzlaw et al., 2015). Thus, understand- glacial maximum, locally dated at c. 24 ka based on the ing the mechanisms of rhyolite genesis in a particular age of glaciated and unglaciated flows at LdM system can inform predictions of the processes and (Singer et al., 2000), has encircled the lake in the central timescales of the formation of a future, potentially large LdM basin and is unprecedented in the southern Andes eruptible silicic magma body. (Fig. 2; see also Table 1; Hildreth et al., 2010; Singer The importance of lower crustal differentiation in et al., 2014). Hildreth et al. (2010) presented several lines producing and in arc settings is well of evidence suggesting that these eruptions are derived recognized (e.g. Hildreth & Moorbath, 1988; Ownby from an integrated silicic magma system, most promin- et al., 2011); it has also been proposed that silicic ently: (1) rhyolite lavas erupted 10–12 km apart have magma is generated in the lower crust by partial nearly identical major and trace element compositions, Journal of Petrology, 2017, Vol. 58, No. 1 87

the last 15 Myr (Hildreth et al., 2010). During the same 100 km period of time, frequent seismic swarms have occurred Santiago San Jose at similarly shallow depths near the Nieblas (rln) and - Barrancas (rcb) rhyolite flows, which are among the -34º Diamante Calabozos youngest in the volcanic field (Fig. 2; Singer et al., Caldera 2014). Initial gravity and magnetotelluric studies also Cerro Azul- Quizapu suggest the presence of a shallow, possibly growing, Talca Puelche magma system beneath the area of deformation at LdM Volcanic Field -36º 7.4 cm/yr Peru-Chile Trench (Singer et al., 2014; Miller et al., 2016). More recent geo- Laguna del Maule detic and geomorphological observations indicate that Concepción Tatara- the rate of uplift and inflation slowed slightly in 2013 (Le San Pedro Chile Domuyo Me´ vel et al., 2015) and that dozens of similar inflation Nevados de Chillan episodes have probably occurred throughout the -38º Holocene (Singer et al., 2015). Argentina The post-glacial eruptive chronology at LdM is cur- rently defined by only four 40Ar/39Ar ages obtained nearly two decades ago (Singer et al., 2000) and the positions of lava flows relative to a paleoshoreline Mocho-Choschuenco -40º marking the highstand of the lake produced when the Puyehue-Cordón Caulle outlet gorge was dammed by the early rle rhyolite flow. Consequently, the age relations of eruptions occurring on opposite sides of the lake have been inferred based -74º -72º -70º -68º only on geomorphological features such as the extent of weathering and degree of cover, hindering Fig. 1. Regional map of the SVZ between 33 and 41 S showing the interpretation of the temporal record. New 40Ar/39Ar the location of Laguna del Maule. Selected frontal arc volcanos 36 (triangles) and caldera systems and silicic volcanic centers and Cl surface exposure ages for late Pleistocene and (dark gray fields) are labeled for reference. The velocity of the post-glacial LdM lavas that refine the eruptive sequence relative to the is calculated are presented in this study. New whole-rock trace elem- using MORVEL (DeMets et al., 2010). ent compositions, Sr, Pb, and Th isotope ratios, and mineral thermobarometry are evaluated in the frame- suggesting that they are derived from a single homoge- work of this new geochronology to examine the tem- neous reservoir; (2) inclusions of mafic magma in rhyo- poral evolution of the rhyolite and magma lavas are common, whereas mafic eruptions compositions. Models of magma evolution spanning have been rare and peripheral since the beginning of the last 150 kyr in the central LdM basin (earlier erup- post-glacial rhyolite volcanism, indicating that a broad, tions are sparse) are used to interrogate the continuity low-density magma body is blocking the ascent of and integration of the LdM magma system, the nature mafic magma. Consequently, the numerous post- and depth of the processes contributing to its evolution glacial silicic eruptions at LdM may represent a high through time, and the implications for the continuing temporal resolution sampling of the evolution of a volcanic unrest. large, shallow magma system. Several geophysical methods document continuing GEOLOGICAL SETTING volcanic unrest within the LdM basin that remains ac- The LdM volcanic field is situated on the tive at the time of this writing. Geodetic data since 2007, crest of the Andes at 36S in the Southern Volcanic obtained by continuous global positioning system Zone (SVZ) of central Chile (Fig. 1). Between 32 and (GPS) and interferometric synthetic aperture radar 37S, the arc is characterized by a gradient in crustal (InSAR), record uplift at a rate in excess of 20 cm a–1, thickness from 35 km in the south to 60 km in the north among the fastest measured at a volcano not actively (Gilbert et al., 2006; Tassara et al., 2006; Tassara & erupting (Fournier et al., 2010; Feigl et al., 2014; Le Echaurren, 2012). This near doubling in thickness cor- Me´ vel et al., 2015). A model of an inflating sill at 5 km relates with a transition from dominantly basaltic an- depth produces the best fit of the measured deform- desite to amphibole-bearing intermediate products and ation pattern, with an estimated volume increase of well-documented gradients in trace element and radio- 3 –1 003–005 km a between 2007 and 2014 (Le Me´ vel genic isotope composition (Hildreth & Moorbath, 1988). et al., 2016). This probably transient rate is one to two Distinctively, the segment of the arc between 34 and orders of magnitude greater than the late Pleistocene to 37S hosts several large Quaternary silicic volcanic cen- Holocene eruptive fluxes at the Southern Volcanic Zone ters in addition to LdM: the Maipo–Diamante Caldera (SVZ) frontal arc centers Mocho–Choshuenco and (Sruoga et al., 2012), the Calabozos Caldera (Hildreth Puyehue–Cordon Caulle (Singer et al., 2008; Rawson et al., 1984; Grunder & Mahood, 1988), Puelche et al., 2015) and the average eruptive flux at LdM over Volcanic Field (Hildreth et al., 1999), and Domuyo 88 Journal of Petrology, 2017, Vol. 58, No. 1

Rio Maule 1600 1800 1700 115 2200 230 1900 240 0 0 N 2100 Cajón Grande de Bobadilla 2828 3122 igcb 2500 Paso 2 2600 rle ig rdne 700 Pehuenche 2000 Cajón Chico de Bobadilla rdno 2874 rep bbc27 2800 rca rle 00 -36.0º igcb 115 bec 2600 igsp 25.7 ka 2500 2400 rdcn 2300 igcb 3.5 ka 19.0 ka 3080 L. Cari rdop rcn rcl Launa 0 00 2400 2680 rddm2300 0 rsl 303 rdsp 2500 0 2889 aam 0000 rsl 2600 2200 asp mpl 303129009000 2767 2800 3.3 ka apv Laguna 2900 3000 ram 2700 00 30 rdam 2600 00 2800 asm del 2500 29 2700 240 igsp apj Maule 2300 0 3175 rdcd 2162 2.1 ka 2883 anc apo rcd acn 2600 mnp aan dlp rdnp 8.0 ka 2300 0 rln 30000 2900 0 280228000 -36.1º rdep 20.0 ka mvc 2400 A. de la Calle 270027027 rdct 2500 2600 mcp 2500250 2600 2400 00 dlp mct 27 ras 2300 2600 2800 rdac 250 2855 rap 5.6 ka 0 22.42900 ka Aroyo de Palacios lveda 2300 3056 ú 2800 2400 2700 2600 2994 A. Sep 2500 1.9 ka rcb L. 2600 Negra 2162 700 Cajón de Troncoso 2 00 a Parva 2802808 l 90000 rcb-d 2 0 280800 Laguna 3037 Fea 30000 2888 2486 2800288000 rroyo de 270272700 rng A 2600 2600 270 rcb-d 00 0 25

00 CHILE 24 -36.2º Arroyo 2300 2200 2000 14.5 ka Puente de Tierra

ARGENTINA 2100 rcb-py Arr oyo Curamili

o 11.4 ka

-70.6º -70.5º -70.4º Central Laguna del Maule Volcanic Field Volcanic Vent Post Glacial Eruptions Pleistocene Eruptions Lava Flow Direction Rhyolite Rhyolite Pumice Rhyodacite Dacite/Rhyodacite Highway 115 Andesite International Boundary 05km Basalt/Mafic Andesite 3.5 Eruption age [ka] Contour interval 50 m Pleistocene ignimbrites Sample Locations all elevations masl igcb - 990 ka; igsp - 1.5 Ma Center of Deformation

Fig. 2. Simplified geological map of the central basin of the LdM volcanic field [after Hildreth et al. (2010)] showing sample loca- tions; unit names and abbreviations are listed in Table 1. Eruption ages are determined by 40Ar/39Ar except for the 36Cl age of unit rdcd; uncertainties associated with the 40Ar/39Ar ages are given in Table 2 and 36Cl data are given in the Supplementary Data. The center of uplift near the southwestern lake shore is an approximate location based on the InSAR model of Feigl et al. (2014). Journal of Petrology, 2017, Vol. 58, No. 1 89

Table 1: Laguna del Maule eruptive units mapped in Fig. 2

Abbreviation* Unit name* Eruption age† aam Andesite of Arroyo Los Mellicos 254 6 15ka acn Andesite of Crater Negro post-glacial anc Andesite north of Crater Negro post-glacial apj Younger andesite of West Peninsula 211 6 34ka apv Older andesite of West Penisula pre-glacial asm Andesite south of Arroyo Los Mellicos post-glacial asp Andesite of Laguna Sin Puerto <35ka bbc Basalt of Volcan Bobadilla Chica 153 6 7ka bec Basalt of El Candado 618 6 36ka dlp Dacite of Laguna del Piojo pre-glacial igcb Ignimbrite of Cajones de Bobadilla (rhyodacite) 990 6 13 ka igsp Ignimbrite of Laguna Sin Puerto (dacite) 1484 6 15 ka mcp Andesite of Crater 2657 post-glacial mct Andesite of Arroyo Cabeceras de Troncoso post-glacial mnp Andesite north of Estero Piojo post-glacial mpl Andesite of Volcan Puente de la Laguna 54 6 21 ka mvc Andesite of Volcan de la Calle 1521 6 65ka ram Rhyolite of Arroyo Los Mellicos post-glacial; >19 ka rap Rhyolite of Arroyo de Palacios 224 6 20ka ras Rhyolite of Arroyo de Sepulveda 19–20 ka rca Rhyolite of Cajon Atravesado 710 6 13 ka rcb Rhyolite of Cerro Barrancas multiple flows; 114–19ka rcb-d Cerro Barrancas Dome Complex (rhyolite) 145 6 15ka rcb-py Cerro Barrancas Pyroclastic Flow (rhyolite) 114 6 11ka rcd Rhyolite of Colada Divisoria 21 6 13ka rcl Rhyolite of Cari Launa <33ka rcn Rhyolite of Cerro Negro 4660 6 56ka rdac Rhyolite of Arroyo de la Calle 200 6 12ka rdam Rhyodacite of Arroyo Los Mellicos post-glacial; >19 ka rdcd Rhyodacite of Colada Dendriforme 80 6 08ka rdcn Rhyodacite of Northwest Coulee 35 6 23ka rdct Rhyodacite of Arroyo Cabeceras de Troncoso 202 6 41 ka rddm Rhyodacite of Domo del Maule 114 6 14 ka rdne Rhyodacite NE of Loma de Los Espejos post-glacial; >19 ka rdno Rhyodacite NW of Loma de Los Espejos post-glacial; >19 ka rdnp Rhyodacite north of Estero Piojo post-glacial rdop Rhyodacite west of Presa Laguna del Maule pre-glacial rdsp Rhyodacite of Laguna Sin Puerto <35ka rep Rhyolite east of Presa Laguna del Maule 257 6 12ka rle Rholite of Loma de Los Espejos 190 6 07ka rle-ig Espejos ignimbrite (rhyolite) post-glacial; >19 ka rln Rhyolite of Colada Las Nieblas Late Holocene rsl Rhyolite south of Laguna Cari Launa 33 6 12ka

*Abbreviations and unit names after Hildreth et al. (2010). †Ages are from Singer et al. (2000), Hildreth et al. (2010), Birsic (2015), and this study; all 40Ar/39Ar ages are calculated relative to the 11864 Ma Alder Creek Sanidine (Jicha et al., 2016).

Volcanic Complex (Miranda et al., 2006; Chiodini et al., calc-alkaline, medium- to high-K compositions typical 2014), each situated in the rear-arc relative to the basalt- of SVZ frontal arc volcanoes. Hildreth et al. (2010) found to andesite-dominated frontal arc volcanoes (Fig. 1). evidence for neither systematic variation in the slab sig- Owing to repeated glaciation and the remote, rugged nature across the volcanic field nor any significant con- terrain, it is not well appreciated that the productivity of tribution of back-arc, alkaline compositions. Basaltic Pliocene to Holocene silicic volcanism in this northern andesite to andesite dominates much of the preserved sector of the SVZ is comparable with that of the Andean eruptive history of LdM, but silicic (dacite–rhyolite) Central Volcanic Zone (Hildreth et al., 1984, 1999). eruptions have occurred throughout the volcanic field Hildreth et al. (2010) documented the most recent 15 during the Pliocene and Pleistocene (Hildreth et al., Myr of volcanic activity at LdM, which comprises more 2010). Two silicic ignimbrites are preserved in the LdM than 350 km3 of lava, tephra, and pyroclastic deposits lake basin (Fig. 2), the 15 Ma two- dacite Sin ranging in composition from basalt to high-silica rhyo- Puerto Ignimbrite (igsp) and the 990 ka biotite rhyoda- lite erupted from at least 130 vents. The Quaternary cite Bobadilla Ignimbrite (igcb)(Birsic, 2015). Of these, eruptions overlie Paleogene to Neogene volcanic and only the Bobadilla caldera structure partially survived volcaniclastic rocks and Pliocene to plutons the subsequent glaciation and erosion. Two middle and sedimentary strata (Nelson et al., 1999; Hildreth Pleistocene rhyolitic lavas are preserved near the north- et al., 2010). LdM volcanic products are of tholeiitic to eastern shore of the lake, the 710 6 13 ka Rhyolite of 90 Journal of Petrology, 2017, Vol. 58, No. 1

Cajon Atravesado (rca) and the 4660 6 56 ka Rhyolite Laguna Sin Puerto (rdsp) are crystal poor. The pheno- of Cerro Negro (rcn). The latter contains the most cryst assemblage is similar to that of the rhyolites but evolved compositions in the volcanic field (Hildreth all lack quartz and contain amphibole. Most rhyodacite et al., 2010). lavas contain fine-grained, partly glassy, basaltic andes- Singer et al. (2000) determined the timing of the last ite inclusions, frequently with quench textures, up to glacial retreat to be between 254 6 12 ka and 232 6 06 40 cm in diameter in the of Colada ka based on 40Ar/39Ar age determinations (recalculated Dendriforme (rdcd) and NW of Loma de Los Espejos to an Alder Creek Sanidine age of 11864 Ma; Jicha (rdno), but more commonly 1–10 cm in diameter. et al., 2016) for four eruptions, including one glaciated Similar inclusions are rare in the Rhyolite of Arroyo Los and three unglaciated lavas at approximately equal ele- Mellicos (ram) mini-dome but have not been found in vation in the LdM basin. This age is consistent with the any other rhyolite. moraine records east of the Andes between 47 and 46S based on 3He, 10Be, and 26Al cosmogenic expos- 40 39 36 ure, 40Ar/39Ar, and 14C ages indicating that the last gla- NEW Ar/ Ar AND Cl AGES AND REVISED cial maximum occurred prior to 23 ka with deglaciation ERUPTION SEQUENCE well under way by 165ka(Kaplan et al., 2004; Hubbard An effort to document the LdM eruptive sequence based et al., 2005; Clark et al., 2009; Hein et al., 2010). The on the tephra stratigraphy and soil 14Cagesiscurrently post-glacial volcanism is concentrated in the LdM lake under way (Fierstein et al.,2012; Sruoga, 2015). basin, producing 36 silicic domes and coulees and doz- However, the construction of a 14C chronology at LdM is ens of explosive eruptions from at least 24 vents encir- challenging owing to a dearth of organic material. cling the lake (Fig. 2; Hildreth et al., 2010; Fierstein et al., Whereas 14C ages typically have lower uncertainties, 2012; Sruoga, 2015). Ten andesite flows emplaced since where suitable material is lacking, 40Ar/39Ar and 36Cl the glacial retreat, primarily along the western lake- ages offer alternative methods to date young volcanic shore, are of subordinate volume. is eruptions. Twenty-six 40Ar/39Ar incremental heating ex- rare since the most recent deglaciation and the young- periments, performed at the WiscAr Geochronology Lab est true basalt is the 61 8 6 3 6 ka basalt of El Candado (see Supplementary Data for details; supplementary (bec) erupted north of LdM (Fig. 2; Hildreth et al., 2010, data are available for downloading at http://www.pet recalculated to an Alder Creek Sanidine age of rology.oxfordjournals.org) yield ages, all but one 11864 Ma; Jicha et al., 2016). containing more than 75% of the released 39Ar, and sup- Silicic eruptions at LdM were explosive and effusive port 12 eruption ages (Fig. 3; Table 2). We attempted to and generally of modest volume (<13km3; Hildreth determine 40Ar/39Ar ages for nearly all post-glacial lavas. et al., 2010; Fierstein et al., 2012). Continuing tephrostra- However, owing to their youth and high atmospheric Ar tigraphic investigations (Fierstein et al., 2012; Sruoga, contents, LdM products commonly yield small fractions 2015) both within the LdM basin and of distal deposits of radiogenic 40Ar (40Ar*). Micropumiceous rhyodacites in Argentina, are not discussed in detail here. However, and commonly vesiculated and glassy andesite flows of particular note, Fierstein et al. (2012) have identified a nearly all produced high 36Ar signals from which 40Ar* voluminous explosive eruption that produced flow and could not be resolved. Dense rhyolitic obsidian more fall deposits up to 6 m thick in Argentina 30 km south and east of LdM accounting for an order of magnitude commonly yields plateau ages; however, only approxi- greater volume than any single event mapped in the mately 50% of such samples produced resolvable ages. 39 central basin by Hildreth et al. (2010). This explosive Recoil of Ar during irradiation of volcanic glass can re- event pre-dates the rle lava flow that dammed the lake sult in spurious ages. This effect is mitigated for the LdM and thus is among the earliest post-glacial rhyolite lavas by a short irradiation time; age plateaux character- eruptions. However, its vent location and eruption age istic of recoil (i.e. decreasing apparent age with increas- remain uncertain. ing step heating temperature) are only sporadically Rhyolite flows preserved in the LdM basin are vitro- observed for sample aliquots subjected to longer dur- phyric and carry 5% modal ; the rhyolite ation irradiation (see Supplementary Data). Several ex- of Arroyo Palacios (rap) and all but the latest of the periments display anomalously high ages in the low or Barrancas complex (rcb) flows are notably aphyric. high temperature steps. However, this behavior is con- Phenocrysts, when present, are dominantly , sistent neither throughout the LdM sample suite, nor be- subordinate biotite, Fe–Ti oxide, sparse quartz, acces- tween aliquots prepared from single samples. The cause sory zircon, apatite, and very rare FeS inclusions in of these discordant steps is not clear, but they account magnetite; several rhyolites also contain scarce amphi- for less than 5% of the gas in single experiments and do bole. With the exception of the rhyodacite of Arroyo de not bias the reported ages. Inverse isochrons for all sam- la Calle (rdac) the rhyodacite lavas are concentrated in ples yield 36Ar/40Ar intercepts within uncertainty of the the western and northwestern basin. They are vitrophy- atmospheric ratio of Lee et al. (2006), indicating that ex- ric to micro-pumiceous and nearly all carry a pheno- cess Ar is not significant. The isochron and plateau ages cryst load of 10–25%, greater than any of the rhyolites; for each experiment are indistinguishable at 2r uncer- only the rhyodacites of the Northwest Coulee (rdcn) and tainty; thus the more precise plateau ages are preferred. Journal of Petrology, 2017, Vol. 58, No. 1 91

40 4.0 Southern Cari Launa Rhyolite (rsl) 30 3.3±1.2 ka 3.5 20 3.0±1.6 ka 3.7±2.1 ka 3.0 10 2.5 5.3±2.9 ka 0 40 39 Ar/ Ar0 = 293.8±8.6 n = 10 -10 2.0 0 0.2 0.4 0.6 0.8 1.0 0.0 0.4 0.8 1.2 1.6 2.0 2.4 50 Espejos Rhyolite (rle) 19.1±0.8 ka 5.0 40 39 19.0±0.7 ka Ar/ Ar0=296.4±2.6 40 n=18 4.0 3 30 17.7±2.5 ka 3.0

20 Ar x 10

40 2.0 Age [ka] Age 10 18.4±1.1 ka Ar/ 36 1.0 19.5±0.9 ka 0 0.0 0 0.2 0.4 0.6 0.8 1.0 0369 60 4.0 Rhyolite East of Presa 26.2±2.6 ka 40 39 50 Laguna del Maule (rep) 3.5 Ar/ Ar0 = 296.5±9.8 25.7±1.2 ka n = 13 40 3.0 30 2.5 20 2.0 25.8±1.3 ka 10 1.5 25.0±3.0 ka 0 1.0 0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Cumulative 39Ar Fraction 39Ar/40Ar

Fig. 3. Example 40Ar/39Ar age spectra and inverse isochrons for units rep, rle and rsl; values for all samples are available in the Supplementary Data. Plateau steps are colored boxes and ellipses; discordant, excluded steps are light gray. All uncertainties are- 6 2r and include the analytical and J uncertainties.

The eruption of the rle flow dammed the northern Early post-glacial eruptions outlet of the lake, causing the lake level to rise to The earliest of the recent silicic units erupted shortly 200 m above its modern level and cutting a prominent prior to deglaciation, forming the rhyolite east of the shoreline into all low-lying older rocks (Hildreth et al., Presa (dam) (rep)at257 6 12 ka in the northwestern 2010; Singer et al., 2015). To constrain better the dur- LdM basin. All subsequently erupted silicic units are ation of the lake highstand, we undertook 36Cl surface unglaciated, including the early voluminous pyroclastic exposure age determinations of the rdcd lava flow, event (Fierstein et al., 2012) and numerous andesite and which overruns the paleoshoreline is several places but rhyodacite flows and domes concentrated in the west- did not produce a resolvable 40Ar/39Ar age, and the ern and northwestern LdM basin. A single unglaciated shoreline itself where it is notched into the igcb ignim- andesite flow (apo) erupted in the south; this glassy, brite along the north shore of the lake (36Cl methods vesiculated lava did not produce resolvable 40Ar*. The and results are in the Supplementary Data). flow is largely buried by lake deposits and a pumice fan, The new age determinations are discussed in con- but apparently was erupted prior to the damming of the junction with the observations made during fieldwork lake. Rhyolite flows erupted on three sides of the lake in in support of the present work and by Hildreth et al. a relatively short time interval at the end of the EPG (2010) to improve the chronology of the post-glacial period: the Sepulveda rhyolite (ras) in the SE [which dir- eruptions. Whereas LdM erupted regularly following ectly overlies the 200 6 12 ka rhyodacite of Arroyo del the last glacial maximum, the rhyolitic volcanism is la Calle (rdac)], the 224 6 20 ka Palacios rhyolite (rap), clustered in two periods of high eruption frequency. and the 190 6 07 ka Espejos rhyolite (rle). An early post-glacial (EPG) group erupted prior to the damming of the outlet gorge at 19 ka. This was fol- Latest Pleistocene to Holocene eruptions lowed by a period of relative calm in much of the lake Volcanic activity waned throughout much of the LdM basin during the latest Pleistocene, with rhyolitic activ- basin following the end-EPG eruptions. The latest ity limited to the Barrancas complex in the SE basin. Pleistocene eruptions were restricted to the Barrancas Finally, silicic eruptions encircled the lake during the center (rcb) on the southeastern rim of the lake basin. Holocene (Fig. 4). An early episode of dome building is dated at 145 6 15 92 Journal of Petrology, 2017, Vol. 58, No. 1

Table 2: Summary of 40Ar/39Ar experiments

40 36 39 Sample no. K/Ca total Total fusion Ar/ Ari 6 2r MSWD Isochron n Ar % MSWD Plateau age [ka] 62r age [ka] 62r age [ka] 62r

Rhyolite of Cerro Barrancas, eat summit flow (rcb) AR-267 581 24 6 21 3015 6 61099 87 6 58 6 of 9 842103 16 6 07 AR-267 580 44 6 31 323 6 33 024 -224 6 99 5 of 10 768085 23 6 09 Combined isochron n ¼ 19: 3010 6 57100 13 6 80 Weighted mean n 5 2: 1719 6 06 Rhyolite of Colada Divisoria (rcd) LdM-249* 506 77 6 32 3052 6 81012 -18 6 39 6 of 10 832065 28 6 23 LdM-249* 518 32 6 27 3007 6 51019 -05 6 10 7 of 8 984027 13 6 25 LdM-249 500 25 6 21 3002 6 56008 09 6 27 6 of 6 1000013 22 6 19 Combined isochron n ¼ 19: 3011 6 34028 03 6 03 Weighted mean n 5 3: 035 21 6 13 Rhyolite of South Cari Launa (rsl) ALDM-13-17 670 17 6 18 2856 6 169116 86 6 57 4 of 7 801160 37 6 21 ALDM-13-17 660 30 6 16 2982 6 114025 32 6 41 6 of 8 962020 30 6 16 Combined isochron n ¼ 10: 2938 6 86063 53 6 29 Weighted mean n 5 2: 065 33 6 12 Rhyodacite of the Northwest Coulee (rdcn) LdM-12-27 146 -12 6 24 2947 6 84154 57 6 45 5 of 7 897 138 35 6 23 Rhyolite of Cerro Barrancas, northern flow (rcb) LdM-210† 531 73 6 19 3088 6 105141 -22 6 24 5 of 6 982243 52 6 27 LdM-210 525 39 6 24 2987 6 162295 26 6 29 5 of 6 979218 27 6 31 LdM-210* 520 106 6 28 2991 6 85040 85 6 59 8 of 9 976034 90 6 24 LdM-210* 526 82 6 32 3015 6 127090 31 6 27 7 of 9 861077 49 6 30 LdM-210 512 56 6 14 3310 6 379063 19 6 19 7 of 7 1000114 57 6 12 Combined isochron n ¼ 27: 2987 6 32157 56 6 13 Weighted mean n 5 4: 147 56 6 11 Cerro Barrancas Pyroclastic Flow (rcb-py) CB-Curamilo A 618 126 6 49 318 6 86047 -88 6 8612of1584112115 613 CB-Curamilo A 588 100 6 51 2949 6 5813166 10 5 of 10 67314113619 Combined isochron n ¼ 18: 2987 6 4812114 6 71 Weighted mean n 5 2: 003 114 6 11 Cerro Barrancas Dome Complex (rcb-d) CB-2 490 137 6 16 2974 6 27080 158 6 36 9 of 9 1000 080 145 6 15 Rhyolite of Loma de Los Espejos (rle) LdM-60 690 207 6 25 2951 6 100131 203 6 76 6 of 8 909117 177 6 25 LdM-60 710 178 6 12 2045 6 1169055 207 6 16 4 of 6 912093 195 6 09 LdM-60 200 182 6 17 2992 6 50023 183 6 16 8 of 9 993021 184 6 11 Combined isochron n ¼ 18: 2964 6 26073 191 6 08 Weighted mean n 5 3: 084 190 6 07 Rhyodacite of Arroyo de la Calle (rdac) LdM-213 205 209 6 18 2941 6 97059 226 6 33 7 of 7 1000062 212 6 15 LdM-213 203 199 6 18 2861 6 120046 224 6 37 5 of 7 939134 188 6 18 Combined isochron n ¼ 13: 2920 6 80113 220 6 26 Weighted mean n 5 2: 128 200 6 12 Rhyolite of Arroyo Palacios (rap) LdM-12-23 620 224 6 20 2934 6 129047 236 6 36 7 of 7 1000 049 224 6 20 Andesite of Arroyo Mellicos (aam) LdM-194 034 323 6 140 3033 6 86087 130 6 99 9 of 9 1000091 288 6 125 LdM-194 031 316 6 70 3027 6 47099 105 6 96 5 of 6 829152 233 6 81 Combined isochron n ¼ 14: 3030 6 41076 108 6 63 Weighted mean n 5 2: 108 245 6 61 Rhyolite East of Presa Laguna del Maule (rep) LdM-12-32 750 247 6 34 2919 6 280082 263 6 58 8 of 8 1000073 250 6 30 LdM-12-32 700 270 6 13 2966 6 106042 263 6 28 5 of 6 834035 258 6 13 Combined isochron n ¼ 13: 2965 6 98060 262 6 26 Weighted mean n 5 2: 056 257 6 12

Weighted mean plateau ages in bold are preferred; 2r uncertainties include the analytical and J uncertainties. *Monitored with the 28.201 Fish Canyon Sanidine (Kuiper et al., 2008); all other experiments were monitored with the 1.1864 Ma Alder Creek Sanidine (Jicha et al., 2016); †high MSWD; not included in weighted mean ka and is followed by an explosive event that produced rhyodacite flow, erupted near the western lake shore; pyroclastic flow deposits extending SE away from the these are the youngest units at sufficiently low elevation lake into Argentina (Fig. 2). A dense vitric clast from this within the lake basin to be subject to, but not affected pyroclastic deposit gave an age of 114 6 11 ka. These by, shoreline erosion. The youngest of the three north- earliest products of the Barrancas complex are exposed ern rcb flows yields an 40Ar/39Ar age of 56 6 11 ka; on its southern and eastern flanks and, therefore, are 40Ar* could not be resolved from either of the underly- not subject to shoreline erosion. Continued activity at ing flows. The rdcd flow yields a whole-rock 36Cl surface the Barrancas complex produced a series of rhyolite exposure age of 80 6 08 ka. The ages of the rdcd and flows, the northernmost of which, along with the rdcd northern rcb flows are consistent with a whole-rock 36Cl Journal of Petrology, 2017, Vol. 58, No. 1 93

(a) 5 km (c) rdne deglaciation rdno rcd lake rcl rle high stand rsl ram rep rdam aam asm apj apo acn rdnp East mnp anc apo rdep rcb summit aan rln ras rdac rcb rap

rng rcb-py Early rcb-d Post glacial ras 25.7 - 19.0 ka rdac rap (b) rdep South rdcd rdnp mnp asp asm rdcn rdsp rcl apj acn rsl anc aan rdcd West rcd rdsp rln asp rdcn mcp mct rle ram rdam rcb-d rcb rdne rdop Latest Pleistocene rng aam rep to Holocene North 14.5 - ≤ 1.9 ka rcb-d rcb-py 30 25 20 15 10 5 0 rcb-py Age [ka]

Fig. 4. Post-glacial eruptive sequence of central LdM basin lavas. Fill colors are the same as in Fig. 2. (a) The distribution of EPG eruptions—those erupted prior to and including the rle flow that dammed the outlet gorge producing the highstand of the lake. (b) The distribution of latest Pleistocene to Holocene eruptions. (c) The relative eruptive sequence constrained by 40Ar/39Ar ages from Singer et al. (2000) and this study; the timing of the drawdown of the lake highstand is constrained by a 95 6 01ka36Cl surface ex- posure age of the highstand shoreline cut into igcb . Black outlined boxes are 40Ar/39Ar and 36Cl ages, with the width corres- ponding to the 2r uncertainty. Gray outlined boxes are inferred eruption age ranges based on field relationships; the widths are set relative to the nominal ages of the constraining events. surface exposure age of 95 6 01 ka for the shoreline Outside the Holocene south–SE rhyolite focus, the cut into igcb in the northern lake basin. rhyodacite of the Northwest Coulee (rdcn) erupted from ThemiddletolateHolocene saw rhyolite eruptions a vent near the crest of the NW basin wall and extends from four centers in the southern and eastern lake nearly down to the current lake level 350 m below. This basin (Fig. 4). A significant explosive eruption from the prominent flow is dated at 35 6 23 ka and is mantled Cari Launa complex (Fierstein et al., 2012)wasfol- by the andesitic cinder ring of Laguna Sin Puerto (asp), lowed by the older of two Cari Launa rhyolite flows which was subsequently intruded by the rhyodacite of (rsl)at33 6 12ka,theRhyoliteofColadaDivisoria Sin Puerto (rdsp). These eruptions likewise emanated (rcd)at21 6 13 ka, and the small rcb floweastofthe from a vent on the crest of the NW basin wall. Two Barrancassummitat19 6 06 ka. Neither the upper- small andesitic fissure eruptions, the andesite of Crater most western rcb flow nor the rhyolite of Colada 2657 (mcp) and the andesite of Arroyo Cabeceras de Las Nieblas (rln) produced resolvable 40Ar*, but on the Troncoso (mct), occurred 6 km west of the SW lake- basis of their similar lack of pumice cover and shore. The ages of these eruptions are not well con- uneroded morphology, they are probably of compar- strained; however, mcp scoria blankets the post-glacial able age to the rcd and eastern summit rcb lavas and rhyodacite south of Estero Piojo (rdep) mini-domes to thus are among the most recent eruptions in the vol- the north, but not the mct craters, indicating that both canic field. are younger than rdep and, although they are at a 94 Journal of Petrology, 2017, Vol. 58, No. 1

30 1000 (a) (d) T-SP 25 Central LdM basin 800 Greater LdM 20 Pleistocene ignimbrites 600 15 Sr [ppm]

Th [ppm] Th 400 10

5 200

0 0 500 1000 (b) (e) T-SP 400 800 T-SP 300 600 K/Rb

Zr [ppm] Zr 200 400

100 200

0 0 15 25 (c) (f)

12 20

9 T-SP 15 Rb/Y 6 La/Yb 10

3 5

0 4550 55 60 65 70 75 80 4550 5560 65 70 75 80

SiO2 [wt. %] SiO2 [wt. %]

Fig. 5. Variation of selected trace elements with SiO2 for lava and pumice erupted in the central LdM basin during approximately the last 150 kyr. Data for the 15 Myr history of the entire volcanic field, including the Pleistocene igcb and igsp ignimbrites (Hildreth et al., 2010; Birsic, 2015) and T-SP (Dungan et al., 2001) are plotted for comparison. The typical 2r uncertainties associated with the central LdM data are smaller than the symbols. The central LdM data show less dispersed ranges and trends relative to the larger LdM volcanic field and T-SP. The REE and Y ratios of the igcb and igsp ignimbrites notably diverge from those of the post-glacial si- licic lavas. Plots of major element variation are available in the Supplementary Data. higher elevation than the high strandline, possibly post- Whereas many major and trace elements, such as date the rle eruption as well (Hildreth et al., 2010). K2O, MgO, Th, U, Rb, and Pb, evolve monotonically with increasing SiO2, several display prominent inflec- tions in variation diagrams (Fig. 5 and supplemen- WHOLE-ROCK GEOCHEMICAL RESULTS tary figures). Between 52 and 60% SiO2, high field Major and trace elements strength elements (HFSE) (except Ti), large ion litho- Lavas erupted during the last 150 kyr in central LdM phile elements (LILE) (except Sr), light REE (LREE), range from basalt to high-silica rhyolite. Primitive lavas, heavy REE (HREE), and Y increase with increasing SiO2. rare throughout the SVZ, are absent from central LdM Between 60 and 68% SiO2, Zr and LREE level off and as indicated by the modest Mg# (53) and low K/Rb TiO2, MREE, Y, and P2O5 begin to decrease. Ba concen- ratios (369–242) of the basalt and mafic andesite sam- trations increase to 65% SiO2 but vary little in the more ples. The major and trace element evolution of central evolved lavas. Between 68 and 70% SiO2, Zr concentra- LdM generally mirrors that of the entire 15 Myr erup- tions begin to decrease and the depletion of Sr with tive history of the larger volcanic field (Hildreth et al., increasing SiO2 becomes greater. 2010) and the frontal arc Tatara–San Pedro complex (T– SP; Dungan et al., 2001). Central LdM trace element Sr and Pb isotope ratios compositions form narrow arrays in elemental variation The Sr and Pb isotope compositions of the central LdM plots compared with the range observed in the volcanic units, measured at the University of Wisconsin–Madison field as a whole (Fig. 5). The Pleistocene LdM ignim- ICP–TIMS Isotope Laboratory [Sr by thermal ionization brites igcb and igsp are notably enriched in rare earth mass spectrometry (TIMS) and Pb by multicollector in- elements (REE), particularly middle REE (MREE), Y, and ductively coupled plasma mass spectrometry (MC-ICP- Zr compared with the post-glacial silicic lavas. MS); see the Supplementary Data for details], display Journal of Petrology, 2017, Vol. 58, No. 1 95

15.75 the SVZ (Nelson et al., 1999). No lava with a comparably Mz intrusions radiogenic Sr isotope ratio has erupted in the central LdM since the middle Pleistocene. The range of central

Pz basement LdM is also similar to, but slightly narrower than, those 15.65 found at the nearby Pleistocene silicic centers including Pb sed the Puelche Volcanic Field (070386–070440; Hildreth 204 Pε-N intrusions et al., 1999) and the Loma Seca Tuff and associated Pb/ 207 15.55 Quaternary frontal arc lavas (070380–070433; Grunder, 1987). MORB SA-N NHRL Th isotopes The Th isotopic compositions, measured by MC-ICP-MS 15.45 at the University of Wisconsin–Madison ICP–TIMS 18.2 18.3 18.4 18.5 18.6 18.7 18.8 Isotope Laboratory (see Supplementary Data for de- 206Pb/204Pb tails), span a narrow range with modest disequilibrium Fig. 6. The 206Pb/204Pb and 207Pb/204Pb ratios of central LdM in both U- and Th-excess (Fig. 8; Table 4). The age- basin lavas (red squares); data are given in Table 3. Also corrected (230Th/232Th) activity ratios of the LdM lavas shown is the Northern Hemisphere Reference Line (NHRL; 0 Hart, 1984), the composition of South Atlantic N-MORB (SA- range from 0773 to 0808, among the lowest yet meas- NMORB; Douglass et al., 1999), Mesozoic (Mz) and Paleogene ured in the SVZ. The rhyolites and rhyodacites display a to Neogene (Pe–N) intrusive rocks (Lucassen et al., 2004), modest U-excess, up to 5%, and a narrow range of Paleozoic (Pz) intrusive and metamorphic basement (Lucassen 230 232 et al., 2004), SVZ sediments (Hildreth & Moorbath, 1988; ( Th/ Th)0 ratios, 0793–0808. The mafic lavas show Lucassen et al., 2010; Jacques et al., 2013), and Quaternary a greater diversity of Th isotopic compositions. The SVZ frontal arc lavas (Davidson et al., 1987; Gerlach et al., 230 232 ( Th/ Th)0 ratios of mafic lavas are nearly all lower 1988; Hildreth & Moorbath, 1988; Hickey-Vargas et al., 1989; McMillan et al., 1989; Jacques et al., 2013; Holm et al., 2014). and have a 50% larger range, 0773–0800, than those of The LdM lavas yield a narrow range of 207Pb/204Pb isotopic the silicic eruptions. Most are in 2–5% Th-excess. ratios compared with the frontal arc edifices and are distinct Quenched mafic inclusions hosted in units rdno and from those of the Paleozoic to Mesozoic basement, indicating rdne and the basaltic andesite lava mpl are in 3–4% that any assimilation was of younger, more primitive crust. 230 232 U-excess and have low ( Th/ Th)0 ratios spanning a similar range to the Th-excess lavas (Fig. 8). limited variation. Ratios of 87Sr/86Sr range from 070407 to 070422, 206Pb/204Pb from 18615 to 18646, 207Pb/204Pb from 15606 to 15622, and 208Pb/204Pb from 38521 to THERMOMETRY AND BAROMETRY 38565 (Fig. 6; Table 3; Supplementary Data Figs A6 and Two-oxide thermometry 207 204 A7). Whereas the Pb/ Pb ratio does not vary coher- The compositions of Fe–Ti oxides were determined by ently with major or trace element composition, higher electron microprobe at the University of Wisconsin– 87 86 206 204 208 204 Sr/ Sr, Pb/ Pb, and Pb/ Pb ratios are corre- Madison (see the Supplementary Data for details). lated with increasing SiO2 (Supplementary Data Fig. A7). In the LdM rhyolites and rhyodacites, the ulvo¨ spinel The late Pleistocene to early post-glacial andesites apj content of magnetite ranges from Ulv13 to Ulv25 and and aam have elevated 87Sr/86Sr ratios similar to those hematite content of ilmenite from Hm25 to Hm31. The of the silicic eruptions, but slightly less radiogenic average Fe–Ti oxide compositions of the apj andesite 206Pb/204Pb and 208Pb/204Pb ratios compared with the flow are Ulv50 and Hm15. Oxides in both the rhyolites more mafic units. In contrast, quenched mafic inclusions and rhyodacites span the compositional range in the northern rhyodacite domes rdno and rdne have observed in the silicic units; however, the highest 87 86 Sr/ Sr ratios similar to those of the basalts and mafic ulvo¨ spinel contents found in rhyolites are limited to the 206 204 208 204 andesite lavas, but higher Pb/ Pb and Pb/ Pb products of the Cari Launa (rcl, rsl) center. 87 86 ratios. The Sr/ Sr ratio of the modest-volume andesite Fe–Ti oxide temperatures calculated using the cali- scoria eruption asp is similar to that of apj, aam,andthe bration of Ghiorso & Evans (2008) are 760–850C for the silicic eruptions, but also has the most radiogenic rhyolites, 796–854C for the post-glacial rhyodacites, 206Pb/204Pb and 208Pb/204Pb ratios of this sample suite. and 760C for the late Pleistocene rhyodacite rddm 87 86 The range of the central LdM Sr/ Sr ratios is not- (Fig. 9). Silicic units yield an fO2 119–132 log units ably narrow compared with regional volcanic centers above the Ni–NiO buffer (NNO). Oxides from the (Fig. 7). The LdM volcanic field as a whole has a wider Younger Andesite of the Western Peninsula (apj) gave a 87 86 range of Sr/ Sr ratios of 070388–070435 and one temperature of 1017 C and fO2 03 log units greater high outlying ratio, 070483, from the 430 ka rhyolite of than NNO. The range of temperatures produced for Cerro Negro (rcn; Hildreth et al., 2010). The multiple oxide pairs from each sample is 35C for all Risco Bayo–Huemul plutonic complex exposed beneath but three samples, commensurate with the 630C un- the Tatara San Pedro volcanic complex contains volu- certainty typically ascribed to the two-oxide thermom- metrically minor domains with 87Sr/86Sr ratios signifi- eter (Ghiorso & Evans, 2008). The later erupted Cari cantly greater (>07050) than those of juvenile lavas in Launa rhyolites and unit rdcd produced temperature 96 Journal of Petrology, 2017, Vol. 58, No. 1

Table 3: Whole-rock Sr and Pb isotopic compositions

Sample Unit 87Sr/86Sr 2SE 206Pb/204Pb 2SE % 207Pb/204Pb 2SE % 208Pb/204Pb 2SE % n

LDM-12-25 aam 070419 000001 18618 000005 15613 000005 38532 000004 2 LDM-12-19 apj 070419 000001 18623 000004 15612 000004 38540 000004 2 ALDM-13-09 asp 070419 000001 18648 000004 15614 000004 38570 000004 2 LDM-12-34 bec 070412 000001 18623 000006 15611 000006 38533 000004 1 LDM-12-31 mnp 070409 000001 18622 000005 15613 000005 38538 000004 1 LDM-12-15 mpl 070408 000001 18623 000008 15621 000007 38550 000004 1 LDM-12-23 rap 070418 000001 18638 000005 15614 000006 38557 000004 2 LDM-13-13 rcb 070419 000001 18636 000004 15615 000004 38557 000004 2 ALDM-13-14 rcb 070420 000001 18633 000005 15612 000005 38549 000004 2 LDM-12-07 rcd 070422 000001 18632 000006 15612 000006 38549 000004 2 LDM-12-08 rcl 070419 000001 18634 000005 15613 000005 38552 000004 1 LDM-12-11 rdac 070420 000001 18638 000005 15614 000005 38553 000004 1 LDM-12-17 rdcd 070418 000001 18640 000004 15613 000005 38557 000004 3 LDM-12-17i rdcdi 070410 000001 18621 000006 15614 000007 38534 000004 1 LDM-12-27 rdcn 070413 000001 18630 000004 15612 000005 38538 000004 2 ALDM-13-10 rddm 070420 000001 18633 000006 15613 000005 38547 000004 1 LDM-12-03 rdne 070421 000001 18636 000006 15616 000005 38551 000004 1 ALDM-13-01 rdnei 070407 000001 18630 000010 15606 000011 38535 000004 1 LDM-12-33 rdno 070420 000001 18635 000003 15613 000003 38551 000004 2 LDM-12-33i rdnoi 070412 000001 18637 000003 15617 000003 38561 000004 1 LDM-12-16 rdnp 070411 000001 18631 000006 15612 000007 38546 000004 1 ALDM-13-08 rdsp 070414 000001 18636 000004 15614 000005 38537 000004 1 LDM-12-32 rep 070420 000001 18637 000004 15612 000005 38550 000004 1 LDM-12-04 rle 070419 000001 18637 000004 15613 000004 38550 000004 3 LDM-12-30b rle p 070420 000001 18637 000004 15613 000004 38554 000004 1 LDM-12-21 rln 070420 000001 18636 000005 15615 000005 38560 000004 1 ALDM-13-17 rsl 070419 000001 18634 000004 15613 000004 38554 000004 2 Standard analyses 2SD 2SD 2SD 2SD NIST SRM-987 071028 000001 30 NBS-981 16940 0004 15496 0004 36720 0011 22 NBS-982 36754 0026 17161 0006 36749 0015 11 BCR-2 070505 000002 18756 0029 15628 0009 38720 0100 5 (Sr), 7(Pb) AGV-2 070402 000002 18861 0041 15619 0005 38535 0044 6 (Sr and Pb)

87Sr/86Sr ratios are reported as measured; age correction is inconsequential for these young samples.

Central LdM SVZ 36º S spreads greater than 60C. The younger Cari Launa lava Basin flow (rcl) and associated pumice cone produced a simi- Rhyolite Greater LdM lar range of temperatures that in aggregate is 812– Rhyodacite Loma Seca Tuff 884C; the lowest temperature in this range is more Basalt - Andesite Tatara - San Pedro Mafic Inclusions than 2r from the mean. Excluding this temperature nar- Puelche Volcanic Field rows the range to 845–884C. Unit rdcd produced a 0.7046 similarly wide range of 823–889C; all calculated tem- peratures are within two standard deviations of the mean. 0.7044 Sr

Amphibole thermobarometry 86 0.7042

Amphibole and plagioclase crystals in five rhyodacite Sr/ 87 lavas (rdac, rdne, rdno, rdcd, and rdcn) were analyzed 0.7040 by electron microprobe at the University of Wisconsin– Madison. The plagioclase compositions are utilized to 0.7038 estimate the magma water content required for the amphibole barometer calibration of Putirka (2016);a 0.7036 0 200 400 600 800 more thorough interrogation of the plagioclase com- Sr [ppm] positions will be the subject of a future contribution. The anorthite content of plagioclase rims ranges from Fig. 7. Comparison of the central LdM basin 87Sr/86Sr as a func- An to An . Using the hygrometer of Waters & Lange tion of Sr content with those of nearby volcanic centers includ- 19 43 ing T-SP (Davidson et al., 1987), the rear-arc Puelche volcanic (2015), the plagioclase rim and rhyodacite whole-rock field (Hildreth et al., 1999), the Calabozos Caldera complex– compositions yield a mean water content for each unit Loma Seca Tuff (Grunder, 1987), and older eruptions through- ranging from 45to50 wt % at 850C and 250 MPa; a out the LdM volcanic field (Hildreth et al., 2010). The regional grand mean of 48 wt % is adopted for the amphibole data are plotted age corrected; the age correction is insignifi- cant for the central LdM lavas and these ratios are plotted as calculations. The Waters & Lange (2015) hygrometer re- measured values. The central LdM lavas show a notably nar- quires an estimate of the crystallization pressure, but is row range compared with these nearby systems. Journal of Petrology, 2017, Vol. 58, No. 1 97

Laguna del Maule Puyehue - Cordon Caulle Llaima (a) (b) 1.0 Quizapu 0.850 Osorno and small Puyehue centers Mafic lava 33º - 41º S historic mafic eruptions Mafic inclusion Rhyodacite 0.825 Rhyolite o 0.9 Rhyolite Glass Th)

232 0.800 Th/ 230 ( 0.8 0.775

equiline equiline 0.7 0.750 0.70.91.11.30.70 0.75 0.80 0.85 0.90 (238U/232Th) (238U/232Th)

Fig. 8. Equiline plots of age-corrected Th isotope activity ratios for central LdM lavas and pumice erupted in the last 150 kyr. (a) The LdM data compared with those measured at other SVZ volcanic systems (Hickey-Vargas et al., 2002; Sigmarsson et al., 2002; Jicha 230 232 et al., 2007; Reubi et al., 2011; Ruprecht & Cooper, 2012). Central LdM lavas have among the lowest ( Th/ Th)0 activity ratios yet measured in the SVZ. (b) Detail equiline plot of the LdM data including the Th-excess mafic lavas and U-excess silicic products and 230 232 rhyodacite-hosted mafic enclaves. The uncertainties in the ( Th/ Th)0 data include those of the ages used to correct the meas- ured ratios for decay since eruption. Dashed tie-lines connect mafic inclusions to their host rhyodacite. relatively insensitive to this parameter. Over a range of distinct differentiation pathways involving diverse crus- 100–900 MPa, the calculated water content varies by tal assimilants and crystallizing assemblages. In the fol- only 015 wt %. Thus, the inclusion of a pressure esti- lowing sections we explore the following: (1) the mate in the hygrometry calculation does not bias the processes that have contributed to the geochemical amphibole barometry. characteristics of the LdM lavas, particularly the sources The LdM amphiboles are pargasite to magnesio- of U- and Th-excess; (2) whether these processes devi- hornblende based on the classification scheme of ate significantly from those inferred at frontal arc volca- Hawthorne et al. (2012). Amphibole formulae based on 23 noes; (3) the processes promoting the more oxygen atoms, pressures, and temperatures are calcu- homogeneous isotopic compositions of the rhyolites lated using the method of Putirka (2016). The equilibrium compared with the mafic samples; (4) the temporal co- melt SiO2 is calculated to assess equilibrium with the host herence of the thermo-chemical evolution of the LdM magma; amphiboles that deviate by more than 4 wt % magma system; (5) the implications for the structure from the host composition, the uncertainty associated and state of the modern magma reservoir. with the equilibrium SiO2 estimate, are not included in the pressure calculations (Putirka, 2016). The resulting dataset Crustal contributions to mafic magmas comprises 12–38 amphibole analyses for each unit and Frontal arc centers in the central and southern SVZ yields average crystallization pressures of 190–250 MPa commonly show relatively narrow ranges of radiogenic with uncertainties of 30–50 MPa (Fig. 9). These pressures isotope ratios, despite trace element evidence for sig- are consistent with those calculated by the less precise, nificant crustal interaction, owing to limited isotopic but magma composition-independent, barometer calibra- contrast between the primary mafic magmas and the ju- tions of Ridolfi et al. (2010) and Ridolfi & Renzulli (2012). venile crust (e.g. Davidson et al., 1987; Dungan et al., Pressure- and magma composition-independent amphi- 2001). Uranium-series isotopes are a sensitive tracer of bole thermometry produces a range of 828–933C, which magma evolution in arc systems as they provide infor- overlaps the two oxide temperatures from the rhyodacite mation about the nature of mantle and crustal compo- lavas, but also extends to higher temperatures. nents, the processes leading to their mixing, and in some cases the timescales of these processes (e.g. Hickey-Vargas et al., 2002; Turner et al., 2003, 2010; DISCUSSION Jicha et al., 2007, 2009; Reubi et al., 2011; Ankney et al., The narrow compositional arrays of the central LdM 2013). Mafic lavas in U-excess are common in arc set- basin lavas suggest a common magmatic origin tings and are often attributed to the flux of slab fluids to (Hildreth et al., 2010). However, divergent correlations the mantle wedge (e.g. Turner et al., 2003). Less com- among radiogenic isotope ratios and inflections in the mon Th-excess continental arc magmas are generally trajectory of trace element variation diagrams suggest thought to reflect a garnet signature inherited from the 98 Journal of Petrology, 2017, Vol. 58, No. 1

Table 4: Whole-rock and glass 230Th–238U compositions

238 232 230 232 230 232 230 238 Sample Unit Age (ka) Th (ppm) U (ppm) ( U/ Th) 2SE ( Th/ Th) 2SE ( Th/ Th)0 2SE* ( Th/ U)0 n

LDM-12-25 aam 254 6 15920 227 0748 0004 0781 0005 0790 0008 1057 1 LDM-12-19 apj 211 6 34804 203 0765 0005 0778 0005 0781 0007 1021 1 ALDM-13-09 asp <35794 207 0792 0005 0790 0005 0790 0005 0998 2 LDM-12-34 bec 618 6 36309 077 0754 0005 0765 0005 0773 0012 1025 1 LDM-12-15 mnp <24 406 109 0814 0005 0800 0005 0798 0008 0981 1 LDM-12-31 mpl 54 6 21 676 165 0742 0004 0770 0005 0788 0020 1062 1 LDM-12-23 rap 224 6 202205 595 0819 0005 0800 0005 0795 0007 0970 1 LDM-13-13 rcb <31880 505 0815 0005 0799 0005 0798 0005 0980 1 ALDM-13-14 rcb 145–562097 561 0812 0005 0799 0005 0798 0006 0982 1 LDM-12-07 rcd 22 6 122059 549 0810 0005 0798 0005 0798 0005 0986 2 LDM-12-08 rcl <332014 541 0815 0005 0799 0005 0798 0005 0979 1 LDM-12-11 rdac 200 6 121999 542 0822 0005 0798 0005 0793 0007 0964 1 LDM-12-17 rdcd 800 6 084 1960 539 0834 0005 0798 0005 0797 0005 0956 2 LDM-12-17i rdcd i 800 6 084 339 088 0784 0005 0798 0005 0798 0005 1017 1 LDM-12-27 rdcn 35 6 231520 412 0822 0005 0799 0005 0798 0006 0971 1 ALDM-13-10 rddm 114 6 14 1822 494 0823 0005 0815 0005 0802 0027 0974 1 LDM-12-03 rdne 257–1901636 436 0809 0005 0799 0005 0797 0007 0985 1 ALDM-13-01 rdne i 257–190539 142 0798 0005 0778 0005 0773 0008 0968 1 LDM-12-33i rdno i 257–190616 167 0823 0005 0783 0005 0774 0008 0941 1 LDM-12-16 rdnp <24 1534 618 0828 0005 0802 0005 0799 0009 0965 1 ALDM-13-08 rdsp <351697 461 0824 0005 0804 0005 0804 0005 0976 1 LDM-12-32 rep 257 6 122342 632 0819 0005 0802 0005 0797 0008 0973 1 LDM-12-04 rle 190 6 072350 632 0816 0005 0803 0005 0800 0007 0980 2 LDM-12-30b rle pum 190 6 072325 620 0810 0005 0808 0005 0808 0007 0998 1 LDM-12-21 rln <31908 514 0817 0005 0800 0005 0800 0005 0980 1 ALDM-13-17 rsl 33 6 122089 556 0808 0005 0797 0005 0796 0005 0985 2 LDM-12-07 rcd glass 21 6 122176 581 0811 0005 0794 0005 0794 0005 0979 1 LDM-12-04 rle glass 190 6 072367 636 0815 0005 0803 0005 0801 0007 0982 1 LDM-12-21 rln glass <32025 542 0812 0005 0798 0005 0798 0005 0982 1 ALDM-13-14 rcb glass 145–562116 566 0812 0005 0794 0005 0793 0006 0977 1 LDM-12-08 rcl glass <352078 560 0817 0005 0801 0005 0801 0005 0980 1 Standard analyses 2SD 2SD BCR-2 587 168 0869 0002 0876 0006 6 AGV-2 601 186 0938 0002 0946 0005 5

230 232 *( Th/ Th)0 uncertainty includes that of the eruption age. mantle or lower crust owing to its affinity for U over Th estimated as the average of that of Palme & O’Neill

(DU/DTh ¼ 23–129; e.g. Rubatto & Hermann, 2007; Qian (2003), will yield Th-excess similar to that measured in & Hermann, 2013). In the SVZ, correlations among the LdM lavas (Fig. 10). However, these low extents of fluid-mobile trace elements, 10Be/9Be, and U-excess in melting favor silica-undersaturated melts inconsistent frontal arc basalts have been interpreted as a slab fluid with the silica-saturated to -oversaturated lavas erupted control of the primary Th isotope signature (Hickey- at LdM. Thus, the Th excess at LdM most probably re- Vargas et al., 2002; Sigmarsson et al., 2002). flects a greater extent of mantle melting and a contribu- However, subsequent U-series studies of several SVZ tion from garnet-bearing crust (GBC). The 207Pb/204Pb centers, including LdM, call into question the ubiquity of ratios of the LdM lavas are distinct from those of the this relationship. The enrichment of fluid-mobile elem- more radiogenic Paleozoic to Mesozoic basement, indi- ents in the SVZ is modest compared with volcanic arcs cating that this crustal component must be relatively globally (e.g. Ba/Th < 300) and is only weakly correlated primitive (Fig. 6; Luccassen et al., 2004). with U-excess (Fig. 10; Supplementary Data Fig. A8). Models of lower crust melting are calculated using ex- Moreover, correlations between fluid-mobile element en- perimental phase equilibria and partition coefficients richment and U-excess can result from crustal assimila- from the literature (see the Supplementary Data for tion rather than variations in the slab fluid signature model parameters). The composition of the lower crust (Reubi et al.,2011). Whereas the addition of slab fluids to is estimated using the global average of Rudnick & Gao 230 232 the mantle wedge plays an important role in promoting (2003); the narrow range of the ( Th/ Th)0 ratios of U-excess at some frontal arc centers, several mechan- the LdM lavas suggest that the initial U/Th ratio of the isms could contribute to their decoupling in the SVZ: (1) crustal component is similar to that observed at LdM, long magma residence (>350 kyr) following the addition and thus the estimated crustal composition is adjusted of the fluid component to the mantle wedge allows the accordingly. Batch melting of GBC (e.g. Berlo et al.,2004; U-excess signature to decay away (Hickey-Vargas et al., Hora et al.,2009) and the formation of garnet during de- 2002); (2) the addition of a Th-enriched sediment melt to hydration melting of initially garnet-free amphibolite the mantle wedge would mitigate the fluid-derived U en- (Wolf & Wyllie, 1993; Ankney et al.,2013)havebeenpro- richment (Jacques et al.,2013). posed to explain Th-excess in continental arc settings. In the absence of significant fluid-derived U enrich- The latter, although appropriate for the large Th-excess ment, 3–6% partial melting of garnet lherzolite mantle observed in Cascade lavas (Jicha et al.,2009; Ankney (e.g. Ottonello et al., 1984), with a composition et al.,2013; Wende et al.,2015), yields large Th-excess Journal of Petrology, 2017, Vol. 58, No. 1 99

-10 0 Rhyodacite lavas (a) (b) rdcn -11 rdcd LdM rdac Loma Seca 100 rdne -12 Rhyodacite Tuff Holocene rdno LdM NNO 2 -13 Rhyolite VTTS EPG O f 200

Log -14 post-Oruanui P [MPa]

-15 2 Bishop Tuff + 300 QFM -16

Glass Mt. -17 400 650 700 750 800 850 900 800 850 900 950 T [ºC] T [ºC]

Fig. 9. Results of mineral thermobarometry for central LdM eruptions. (a) T–fO2 plot for central LdM silicic eruptions. Fields show the range of temperatures and oxygen fugacities for the Loma Seca Tuff (Grunder & Mahood, 1988), Bishop Tuff (Hildreth & Wilson, 2007), Glass Mountain rhyolites (Metz & Mahood, 1991), post-Oruanui rhyolites (Sutton et al., 2000) and the Valley of Ten Thousand Smokes rhyolites (VTTS; Hildreth, 1983). Reference T–fO2 curves for the nickel–nickel oxide buffer (NNO) and 2 log units above the quartz–fayalite–magnetite buffer (QFM þ 2) are shown, illustrating the highly consistent T–fO2 buffering of the LdM erup- tions. (b) Temperatures and pressures derived from amphibole compositions for LdM rhyodacite lavas. The pressure calculation assumes a magma with 48wt%H2O based on plagioclase hygrometry (Waters & Lange, 2015). Each point is a single spot analysis and has uncertainties of 630C and 6160 MPa (Putirka, 2016). The bars on the left of the plot are the average pressure and associ- ated uncertainty for each unit. The pressures of the Holocene lavas are nominally 50–60 MPa less than, but within uncertainty of, those of the EPG units. and HREE depletions inappropriate for the SVZ between mantle-derived melts and the continental (Supplementary Data Fig. A3). Mixing of 10% partial crust. Moreover, these processes vary little from those melts of garnet-bearing crust and mantle reasonably re- inferred at frontal arc centers throughout the SVZ (e.g. produces the range of Th-excess and REE compositions Davidson et al., 1987; Hildreth & Moorbath, 1988; found at LdM; however, the presence of U-excess mafic McMillan et al., 1989; Dungan et al., 2001; Costa & lavas requires an additional explanation (Figs 10 and 11). Singer, 2002; Jicha et al., 2007). Thus, whereas the con- LdM mafic lavas in U-excess could be interpreted as centration of rhyolite at LdM is exceptional, the underly- reflecting the slab fluid signature only partially over- ing mafic magmatic processes are not. printed in the lower crust. However, these samples are enriched in incompatible elements relative to the basalts Shallow vs deep origin of rhyolite and mafic andesites in Th-excess, indicating that the The LdM silicic lavas are depleted in Ti, P, Sr, and Y, U-excess mafic lavas have experienced greater inter- have negative Eu anomalies, and have lower Dy/Yb action with a crustal component. In contrast to garnet ratios relative to the andesites (Fig. 5; Supplementary production by amphibolite dehydration, the formation of Data Fig. A5). These trends indicate a shift in the differ- clinopyroxene during the melting of plagioclase- and entiation regime from that of mafic magmas primarily amphibole-bearing crust (garnet-free crust; GFC) can influenced by assimilation of crustal melts. Annen et al. produce U-excess (Fig. 11; Beard & Lofgren, 1991; Berlo (2006) suggested that the majority of compositional di- et al.,2004). Holocene intermediate lavas at T-SP were versity of volcanic rocks is imparted by lower crustal produced, in part, by the melting of hornblende-bearing processes. This model is inconsistent with the relatively mafic intrusions similar to T-SP xenoliths (Costa & shallow crystallization pressures determined by amphi- Singer, 2002). A 10% dehydration melt of this material bole barometry at LdM. However, it is possible that the yields 6% U-excess, commensurate with the range observed in the LdM lavas (Fig. 10). amphibole is late crystallized and does not capture the Mixing among the mantle, GBC, and GFC end- high-pressure differentiation history of the silicic mag- members, each produced by 10% partial melting, can mas. Differences in phase equilibria and the compos- explain the Th isotope and trace element diversity of ition of potentially assimilated rocks between the deep the LdM mafic lavas. Variation of the Th isotope ratios and shallow crust would impart predictable, divergent with the Zr/Th and La/Yb ratios forms offset arrays with geochemical trends during the generation of silicic the largest Th-excess, found in units mpl and aam, magma that are compared with the LdM compositions associated with higher La/Yb and lower Zr/Th ratios. to judge the plausibility of differentiation in the lower vs This offset is consistent with variable mixing, 5–30%, of upper crust. the GBC and mantle melts. Additional mixing with a We utilize Rhyolite-MELTS (Gualda et al., 2012) 10% partial melt of GFC yields the range of U-series dis- to simulate fractional crystallization of an andesitic equilibrium observed in the LdM mafic lavas (Fig. 11). LdM parental magma at a range of pressures Thus, despite a relatively limited range in isotopic com- (150–1050 MPa), initial water contents (1–6 wt %), and positions, the LdM lavas reflect extensive interactions fO2 buffers (QFM to QFM þ 2, where QFM is quartz– 100 Journal of Petrology, 2017, Vol. 58, No. 1

350 water content. High pressures, water contents and (a) Quizapu Villarica SEC reducing conditions promote the early stabilization of LdM Puyehue 300 pyroxene at the expense of plagioclase and magnetite, equiline Llaima Puyehue SEC Villarica Osorno producing large depletions in MgO over a narrow range 250 of SiO2, inconsistent with the LdM compositions (Fig. 12). Moreover, Gaulda & Ghiorso (2013) argued that the 200 increasing stability of quartz with depth precludes the generation of rhyolite by high-pressure fractional Ba/Th 150 crystallization. 100 MELTS is not well calibrated for hydrous intermedi- ate to silicic compositions saturated in amphibole. 50 However, in this case, the SiO2/MgO ratio of LdM amphibole (26–34) is between those of orthopyroxene (2–3) and clinopyroxene (33–44) predicted by MELTS 0.6 0.8 1.0 1.2 1.4 1.6 1.8 such that the crystallization of either two or (238U/ 230Th) 0 amphibole would have a similar impact on the magma 0.84 SiO2/MgO ratio. Whereas some model misfit may result (b) from the prediction of pyroxene rather than amphibole

quiline e crystallization, the agreement between the MELTS mod- 0.82 eling and amphibole barometry indicates that the sup- LdM silicic pression of plagioclase and magnetite crystallization is 0 Garnet 2 3 4610 15 lavas 0.80 lherzolite the more important factor. Thus, MELTS simulations of Th) melting hydrous systems must be interpreted with caution, but 232 30 can yield useful first-order phase equilibrium con- Th/ 0.78 5101520 1510 5 230 straints even when amphibole is present. ( Garnet-bearing crust melting Garnet-free The physical plausibility of a viscous rhyolite magma 0.76 crust melting ascending >30 km through the crust is questionable (e.g. Rubin, 1995). Even if it were possible, the similarity of the rhyolite 87Sr/86Sr ratios to those of the mafic 0.74 0.65 0.70 0.75 0.80 0.85 0.90 and rhyodacite lavas (Fig. 7) weigh against a deep crust (238U/232Th) origin. Following differentiation in the lower crust, Sr- depleted rhyolite would then traverse the crustal col- Fig. 10. Sources of U-series disequilibrium in central LdM lavas. (a) Plot of SVZ U-series disequilibrium data for mafic umn that includes highly radiogenic Paleozoic to lavas compared with the Ba/Th ratio, an indicator of fluid en- Mesozoic rocks (Lucassen et al., 2004; Supplementary richment. Volcanic centers, including small eruptive centers Data Fig. A6). The inevitable assimilation of even small (SEC) associated with larger edifices, are listed in the legend in geographical order from north (Quizapu) to south (Osorno) amounts (<5%) of this material would produce higher 87 86 along the arc (Fig. 1). Some centers display evidence of cou- and more variable Sr/ Sr ratios in the rhyolites than pling between fluid enrichment and U-excess; however, this observed. The more radiogenic 87Sr/86Sr ratios, correlation may also result from crustal overprinting of the >07046, of the mid-Pleistocene rcn rhyolite erupted in slab signature (Reubi et al., 2011). The range of Ba/Th in the Th-excess lavas is similar to that in U-excess and thus a strong the eastern LdM basin and the most-evolved domains coupling between fluid enrichment and U-series disequilibrium of the Miocene plutonic complex beneath T-SP (Nelson is not evident in the SVZ. Data sources: Sun (2001), Hickey- et al., 1999; Hildreth et al., 2010) potentially reflect as- Vargas et al. (2002), Jicha et al. (2007), Reubi et al. (2011) and Ruprecht & Cooper (2012). (b) The U-series disequilibrium ex- similation of this material; however, the modestly radio- pected during melting of the garnet-bearing mantle, garnet- genic, homogeneous 87Sr/86Sr ratios of the post-glacial bearing lower crust, and garnet-free crust (see the rhyolites do not. Taken together, the isotope ratios of si- Supplementary Data for model parameters). The Th-excess ap- parent in the mafic LdM samples (red squares) can be pro- licic LdM lavas, the incongruity between the predicted duced by melting with residual garnet in either the mantle or phase equilibrium and the LdM major element compos- lower crust. U-excess in several mafic andesites and the silicic itions, and shallow crystallization pressures recorded lavas reflects the overprinting of the garnet signature by partial by amphibole barometry rule out generation of the LdM melting of garnet-free crust rather than U enrichment imparted by a fluid (see text). rhyolites in the lower crust.

Shallow hybridization and fractional fayalite–magnetite) to evaluate the conditions in which crystallization the LdM rhyolite magma formed. Each model is cooled The narrow range of Th isotope ratios and uniform from the calculated liquidus to c. 700C, depending on U-excess of the silicic lavas contrast with the more var- model convergence at low melt fractions. The variation ied mafic compositions (Fig. 8). Fractionating Th from U of SiO2 and MgO of the LdM lavas is best reproduced in the upper crust to produce the silicic compositions by shallow, oxidizing conditions and a moderate initial from a parental melt in Th-excess is not Journal of Petrology, 2017, Vol. 58, No. 1 101

1.15 (a) (b)

1.10 10% GFC melt 10% GFC 50 50 melt 1.05 30 0 LdM 30 LdM silicic 50 silicic

Th) lavas 0 10 lavas 5 0 equiline 10 equiline 230 1.00 3 30

U/ 10% mantle 238 ( 10 melt 10 10% 0.95 10% GBC 5 mantle 5 10 melt 10 melt 10% 20 20 GBC 50 30 30 50 0.90 melt

0.85 10 20 30 40 50 0102030 Zr/Th La/Yb

(c)Mantle melt + 30% GBC melt (d) Mantle melt + 5% GBC melt

100

10 % GFC melt sample/chondrite 10 30 50 1 La Ce Nd Sm Eu Dy Yb La Ce Nd SmEu Dy Yb

Fig. 11. A mixing model to explain the variation of U-series disequilibrium and the trace element composition of the mafic LdM lavas. The mixing endmembers are 10% melts of garnet lherzolite mantle, garnet-bearing crust (GBC), and garnet-free crust (GFC) (see the text and Supplementary Data). (a) and (b) show the variation of U-series disequilibrium with the Zr/Th and La/Yb ratios pro- duced by first mixing mantle and GBC melts. Subsequent mixing with a 10% melt of garnet-free crust produces the range of Th- and U-excess observed in the LdM mafic samples (red squares). The offset arrays of LdM data are consistent with varying mixing proportions of the mantle and GBC end-members. (c) and (d) show chondrite-normalized (Sun & McDonough, 1989) REE patterns produced by 10%, 30%, and 50% mixing of the GFC endmember with a melt composed of 5% or 30% mixing of GBC with the man- tle melt, compared with those of the mafic LdM lavas (gray field). straightforward. Crystallization of major phases will not several volcanoes in the Andes, Cascades, and Alaska significantly increase the U/Th ratio, but accessory (Garrison et al., 2006; Jicha et al., 2007; Turner et al., phases such as apatite, titanite, allanite, and monazite 2010; Ankney et al., 2013). This transition has variously have greater leverage (Berlo et al., 2004). Of these, only been ascribed to mixing with a U-excess endmember apatite is common at LdM. Rare, possibly xenocrystic, derived from small degrees of partial melting with re- titanite has been recovered by heavy liquid separation sidual accessory phases, hydrothermal alteration of from the large, early tephra eruption, but not from any assimilated wallrock, and variation in the contribution other LdM rhyolite; neither allanite nor monazite are of a subduction component through time. The garnet- present. The crystallization of sufficient apatite or titan- free crustal component evident at LdM offers an alter- ite to produce U-excess from a Th-excess mafic magma native explanation. The requirement of garnet in the is not consistent with the P2O5 and MREE compositions production of Th-excess limits this process to the lower- of the LdM lavas: fractionation of 03% titanite most crust. Thus, only rapidly ascending magmas

(DTh ¼ 187, DU ¼ 7, DDy ¼ 935, DYb ¼ 393; Bachmann would preserve a garnet-derived Th isotope signature. et al., 2005)or32% apatite (DTh ¼ 282, DU ¼ 19; Those that stall in the middle to upper crust and further Condomines, 1997) is required to produce the observed differentiate will have greater opportunity to interact change in the U/Th ratio. The crystallization of these with GFC and acquire U-excess. Amphibole, common in phases in this quantity would decrease the Dy/Yb ratio arc crust, is produced both by direct crystallization and by a factor of seven and the P2O5 composition by 14wt by reaction between clinopyroxene and ascending hy- %, respectively; both are approximately four times drous melt. Costa et al. (2002) advocated the latter greater than the variation observed in the central LdM mechanism for the generation of amphibole beneath lava compositions. Thus, the crystallization of accessory T-SP and it also probably occurs at LdM. The subse- phases cannot account for the U-excess observed in the quent melting of amphibole-bearing crust has been pro- silicic lavas. posed as an important source of melt and volatiles in The eruption of mafic magma in Th-excess and volcanic arcs more generally (e.g. Davidson et al., 2007, evolved magma in U-excess has been observed at 2013); thus, the production of clinopyroxene during 102 Journal of Petrology, 2017, Vol. 58, No. 1 amphibole dehydration may be an under-appreciated 4 Model Conditions source of U-excess in intermediate to evolved continen- 210 MPa, 3% H2O, QFM+2 tal arc magmas. 210 MPa, 3% H2O, QFM The evolution of the major and many trace element 3 210 MPa, 5% H O, QFM+2 compositions from the andesitic to silicic magmas is con- 2 600 MPa, 3% H O, QFM+2 sistent with the fractionation of the plagioclase þ 2 amphibole þ biotite þ Fe–Ti oxide þ apatite 6 zircon as- 2 semblage observed in the rhyodacite and rhyolite lavas.

The saturation of zircon yields prominent inflections in the %] [wt. MgO evolution of the Zr concentration (Fig. 5); deviations from 1 the expected closed-system evolution would favor more extensive open-system processes. Zr and Th are similarly (a) incompatible in major phases and thus, prior to zircon sat- 0 uration, fractional crystallization would produce compar- 55 60 65 70 75 80 able enrichments in both elements. In central LdM, the SiO2 [wt. %] 1.0 modest difference in the Zr concentrations of the rhyoda- mt cite and andesite lavas is incongruent with the two- 0.9 cpx difference in the Th concentrations. 0.8 We first consider a model of zircon-free fractional 0.7 crystallization of an andesite parental magma utilizing a plag range of Zr partition coefficients, the anhydrous mineral 0.6 opx assemblage predicted by the best-fit MELTS model (Fig. 0.5 12), and a hydrous mineral assemblage in which amphi- melt phase fraction 0.4 bole crystallizes in place of pyroxene (Table 5). None of bt+ 0.3 these fractional crystallization pathways are able to pro- qtz duce the variation in Zr composition of the intermediate 0.2 LdM lavas (Fig. 13). The zircon saturation temperature 0.1 210 MPa, 3% H2O, QFM+2 (b) of most of the post-glacial rhyodacites is less than but 0 within uncertainty of the two-oxide temperature, indi- 1050 950 850 750 cating they may have been zircon saturated—based on 1.0 opx the zircon saturation model of Watson & Harrison 0.9 cpx (1983); none are zircon saturated using the model of 0.8 Boehnke et al. (2013). Thus, the Zr contents of the rho- 0.7 dacite lavas could be produced by fractional crystalliza- plag tion including a small but increasing modal per cent 0.6 mt zircon or could reflect open-system processes. 0.5 The two-oxide temperature of the andesite apj is 0.4 gt+

phase fraction melt qtz 1017 C, several hundred degrees higher than the zircon 0.3 saturation temperature of this lava (Watson & Harrison, 0.2 1983; Boehnke et al., 2013). The onset of zircon satur- ation during cooling is evaluated by combining the 0.1 600 MPa, 3% H2O, QFM+2 (c) major element composition–crystallinity–temperature 0 relationship predicted by MELTS with a zircon-free frac- 1050 950 850 750 tional crystallization model of the Zr content. The crys- T [C] tallizing andesite magma saturates zircon after cooling Fig. 12. Comparison of rhyolite-MELTS (Gualda et al.,2012)frac- c. 260 C, resulting in 47% crystallization and reaching a tional crystallization simulations with the SiO2–MgO variation of maximum Zr concentration of 305 ppm. This Zr content central LdM lavas to evaluate the effect of crystallization at a is 15% greater than that of the central LdM rhyodacites, range of pressure, H2O, and fO2 conditions. (a) Four representa- tive MELTS simulations. These models are not exhaustive of the indicating that they evolved under dominantly zircon- range of conditions considered but rather were selected to illus- undersaturated conditions (Fig. 13). trate the effect of changes to each parameter (see text). The best- Moreover, the conclusions of this model are consist- fit model (thick black line) involves a low-pressure, moderate water content, and oxidizing conditions, consistent with the min- ent with the amphibole thermometry. The amphibole eral thermobarometry (Fig. 9). Higher pressures, water content, temperatures and equilibrium melt SiO2 compositions and more reducing conditions produce significant depletions in define an SiO2–temperature evolution that deviates MgO at intermediate SiO2 contents that strongly contrast with somewhat from the relationship predicted by MELTS. the LdM data. (b) and (c) illustrate the contrasting crystallizing as- semblage produced at 210 and 600 MPa. Higher pressures, as Nevertheless, the comparison of the zircon saturation well as high H2O and more reducing conditions, stabilize pyrox- temperatures of the LdM lavas and the amphibole crys- ene early at the expense of magnetite and plagioclase and pro- tallization temperatures indicates that zircon was not duce MgO-depleted magmas. Mineral abbreviations: mt, magnetite; cpx, clinopyroxene; opx, orthopyroxene; plag, plagio- saturated in the LdM magma until it reached c. 70% clase; bt, biotite; gt, garnet; qtz, quartz. Journal of Petrology, 2017, Vol. 58, No. 1 103

SiO2 (Fig. 13). Thus, whereas some LdM rhyodacites in the rhyodacite lavas records the shallow mixing and may have saturated zircon prior to eruption, it was a mingling of mafic and silicic magmas. Moreover, late-crystallizing phase, and the rhyodacite Zr contents plagioclase phenocrysts display a range of textures are primarily the result of open-system processes rather ranging from relatively homogeneous to complexly than zircon fractionation. The rhyolite compositions are zoned, including resorption surfaces with overgrowths consistent with an additional 20–35% crystallization of and mottled and sieved cores reflecting varied and an intermediate hybridized magma, assuming fraction- complex thermal histories (Fig. 15). In contrast, plagio- ation of the mineral assemblage observed in the silicic clase in the rhyolite lavas is only weakly zoned; this is LdM lavas (Fig. 13; Table 5). probably the result of efficient melt extraction from the The array of rhyodacite Th–Zr compositions does not zones of magma hybridization, followed by a limited de- readily implicate an LdM lava composition as the silicic gree of cooling and crystallization prior to eruption. mixing endmember. It is relatively enriched in most in- Taken together, the inferred trace element and isotopic compatible trace elements, similar to the LdM rhyolites, composition of the silicic endmember and the outcrop- but not depleted in Zr (Fig. 13). The isotopic compos- to mineral-scale textures of the rhyodacite lavas favor ition of the LdM lavas weighs against significant remo- self-assimilation—hybridization of intruding mafic to bilization of existing silicic crust, such as the plutons intermediate magma with the post-glacial silicic reser- beneath T-SP or the Pleistocene LdM ignimbrites. voir including the resorption of zircon, rather than as- Whereas the mafic LdM lavas span the entire range of similation of the upper crust (Figs 13 and 14). Sr and Pb isotopic compositions measured in the cen- Extensive rhyolitic magma was probably not avail- tral basin, reflecting the diversity imparted by lower able during the early growth of the LdM system. Thus, crustal interactions, and the rhyodacites nearly so, the the initial stages of magma reservoir development may rhyolites exhibit more homogeneous isotope ratios des- have involved remobilizing remnants of mid- pite the wide spatial distribution of vents (Fig. 14). Pleistocene episodes of silicic magmatism and shallow Significant contributions from the modestly more radio- silicic intrusions or production of silicic magma by genic and isotopically diverse upper crust would yield closed-system processes (Fig. 13). Geochemical evi- higher, and probably more variable, 87Sr/86Sr ratios in dence of such magma has not yet been identified and the LdM silicic lavas than observed. they may have never produced an eruption. As the The silicic end-member could be the product of rela- post-glacial silicic system grew progressively larger, the tively closed-system differentiation (Fig. 13). However, assimilation of young, hybridized rhyolite overtook any no magma with a composition consistent with this evo- contribution of the older material or highly fractionated lution has erupted in the LdM since the 990 ka Bobadilla magma. Through self-assimilation, the increasing size ignimbrite. Moreover, a silicic mixing endmember and homogeneity of the LdM magma system is a derived from closed-system differentiation would in- coupled and self-reinforcing process. herit the more varied isotopic ratios of the mafic lavas and would not promote the increasingly homogeneous Temporal evolution of the LdM magma system isotopic ratios in the more evolved magmas. The temporal and spatial distribution of LdM eruptions Instead, the high temperature of the intruding mafic favors a laterally integrated shallow silicic magma sys- magma could promote the resorption of zircon during tem (Hildreth et al., 2010) and offers clues to its struc- magma hybridization, thereby enriching the Zr content ture and variations in the magmatic focus through time. relative to the rhyolitic magma. Zircon has not been Eruptions in the southern and eastern LdM basin were identified in thin sections of the LdM rhyodacites so the dominantly rhyolitic, excepting the apo andesite, presence of rare, partially resorbed zircon cannot be throughout post-glacial times, whereas volcanism in confirmed. However, the abundance of mafic inclusions the NW is characterized by a wider range of

Table 5: Partition coefficients and phase proportions used in fractional crystallization models

Phase Partition coefficients Fractionated phase proportions (%)

Zr Th Intermediate Intermediate Silicic anhydrous hydrous plagioclase 0001–001 00006 59 59 69 orthopyroxene 0026–014 004–022 5 clinopyroxene 013–041 004–029 20 amphibole 023–093 001–025 25 10 magnetite 0025–035 005–042 16 16 5 biotite 005 001–0515 apatite 001 108–282 090909 zircon* 6–18 007 007 008

*The Zr content of zircon is assumed to be stoichiometric. Data sources: Luhr & Carmichael (1980); Bacon & Druitt (1988); Dunn & Sen (1994); Ewart & Griffin (1994); Sisson (1994); Brenan et al. (1995); Bindeman et al. (1998); Villemant (1988); Sano et al. (2002); Blundy & Wood (2003); Bachmann et al. (2005). 104 Journal of Petrology, 2017, Vol. 58, No. 1

400 1200 (a) LdM (b) amphibole GBC ig. thermometry melt 1100 LdM lava zrc FC, 0.07% zrc MELTS model saturation T 300 1000 80 zrc-free FC 60 20 zrc 10 200 GFC resorption 900

20 T [°C]

Zr [ppm] Zr melt

W&H(1983) 30 800 100 FC, 0.08% zrc B(2013) mantle Miocene plutons 700 melt zircon undersaturated zircon saturated zrc under-saturated zrc saturated saturation not determined 0 600 0 102030 58 63 68 73 Th [ppm] SiO2 [wt. %]

Fig. 13. Comparison of fractional crystallization and magma mixing contributions to the upper crustal genesis of the LdM silicic lavas and evaluation of zircon saturation during magma differentiation. (a) The variation of Th and Zr concentrations in central LdM lavas. The trace element compositions of the mafic lavas are dominated by mixing between partial melts of the mantle, GBC, and GFC (Fig. 11). Zircon-free fractional crystallization (zrc-free FC) of a parental andesitic magma produces an enrichment in Zr greater than observed in the LdM rhyodacites; the dashed lines show the range of models produced using the low and high partition coeffi- cients reported in the literature and hydrous vs anhydrous fractionating assemblages (Table 5). The temperature evolution of the best-fit MELTS simulation in Fig. 12 predicts that the magma system will saturate in zircon at 305 ppm Zr, indicating that the flat Zr evolution of the rhyodacites is not due to the fractionation of a small modal fraction of zircon. Instead, the rhyodacite compositions are most consistent with mixing between intermediate and silicic LdM magmas (green line), the latter enriched in Zr by the resorp- tion of zircon (green diamond). The rhyolite compositions are consistent with an additional 20–35% crystallization of a hybridized intermediate magma. (b) The SiO2–temperature evolution of the MELTS model calculated from the amphibole compositions com- pared with the SiO2–zircon saturation temperature relationship of the LdM lavas—calculated using the calibration of Watson & Harrison (1983)—and predicted by the MELTS fractional crystallization model using the zircon saturation calibration of both Watson & Harrison (1983) [W&H(1983)] and Boehnke et al. (2013) [B(2013)]. Both the model and mineral data predict that the magma saturates in zircon at c. 70% SiO2, consistent with the inflection in the whole-rock SiO2–Zr variation (Fig. 5). compositions (Fig. 4). The common andesite eruptions the EPG rhyodacites, although this difference is within in the west and NW during EPG time suggest that the the uncertainty of the barometer calibration (Fig. 9). upper crustal magma system was thinner there relative These results suggest, but cannot prove, that the lateral to the south. Similarly, the numerous rhyodacite erup- growth of the LdM system may have been accompa- tions in the NW carry abundant, large mafic inclusions, nied by the shallowing of active magmatism. whereas they are rare and small in the lone post-glacial Whereas spatial distinctions in the distribution of rhyodacite eruption in the south, rdac. Taken together, mafic and silicic eruptions are readily apparent, com- magmatism in the central LdM has been focused in the positional differences among the post-glacial rhyolites southern basin since before the last glacial maximum, are subtle, but coherent in time rather than with vent lo- resulting in a well-developed mush and a preponder- cation. Holocene rhyolites are enriched in Y and MREE ance of rhyolitic eruptions. compared with the EPG rhyolites (Fig. 16). Two-oxide In the NW, the most recently erupted rhyolite is the temperatures vary similarly. The eruption temperature 190karle flow; subsequent eruptions of any compos- ranges of the EPG (737–801C) and Holocene (781– ition are scarce until the late Holocene (Fig. 4). The most 850C) rhyolites overlap; however, the Holocene tem- recent northern eruptions, units rdcn and rdsp, peratures are consistently at the higher end of the total occurred after a local hiatus of as much as 15 kyr. These range, suggesting an increase in magma reservoir tem- geographical differences in the eruption frequency and perature with time (Fig. 16). That the earlier erupted physical and compositional characteristics of the erup- rhyolite is cooler and more evolved precludes linking tive products indicate that the crystal mush, well de- the EPG and Holocene compositions by a progressive veloped to the south, either thins or is discontinuous differentiation or mixing process. beneath the NW portion of the lake basin. Renewed vol- The variation of most trace elements in the rhyolites canism in the NW during the Holocene produced units defines a single liquid line of decent; in contrast, the rdcn and rdsp, suggesting a recent expansion of the Holocene enrichments in Y and MREE define opposing magmatic footprint at LdM and potentially lateral trends with SiO2 compared with the earlier erupted growth of the active silicic magma system. The amphi- rhyolites. These trace elements show flat or decreasing bole crystallization pressures of the Holocene rdcn and trends with SiO2 in the EPG rhyolites but increasing rdcd lavas are nominally 50–60 MPa less than those of trends in the Holocene rhyolites (Fig. 16) and thus are Journal of Petrology, 2017, Vol. 58, No. 1 105

0.7044 magmatic systems, the temporally coherent, spatially (a) extensive rhyolitic eruptions imply the extraction of chemically distinct magma from a long-lived, compos- 0.7043 LdM ignimbrites itionally evolving, upper crustal source region. Miocene Long-term variations in rhyolite composition, tem- plutons perature, and mineralogy can be driven by variations in Sr 86 0.7042 the lower crust temperature in response to the basalt Sr/ 80 60 40 20 flux from the mantle and changes in the supply of slab 87 fluids (Deering et al., 2008, 2010). However, at LdM, the 80 relatively short duration of rhyolitic volcanism and 0.7041 60 40 20 nearly invariant fO2 buffering indicate that the subtle differences in trace element composition and tempera- 0.7040 ture are more probably related to the upper crust proc- 0 100 200 300 400 500 600 esses of rhyolite differentiation and extraction. Hildreth Sr [ppm] (2004) proposed that trace element variations among 0.7043 (b) broadly homogeneous rhyolites can reflect the variable stability of accessory phases. Similarly, Barker et al. (2014, 2015) attributed the diversity of post-Oruani sili- cic magma compositions at Taupo volcano, in part, to 20 40 60 0.7042 the resorption of amphibole, clinopyroxene, and zircon.

Sr At LdM, extraction of a volatile-rich rhyolite would 86 80 leave behind a relatively water-poor cumulate mush Sr/

87 (Wolff et al., 2015). The repeated intrusion of mafic 0.7041 60 magma would promote the resorption of amphibole or 20 40 late crystallized, cryptic titanite, resulting in MREE- and Y-enriched magma (e.g. Deering et al., 2011). Thus, the eruption of compositionally distinct rhyolites over time 0.7040 may reflect long-term changes in the phase equilibrium 18.61 18.62 18.63 18.64 and temperature of the plutonic mush induced by the 206Pb/204Pb aggregate effect of at least 26 kyr of mafic intrusions Fig. 14. The effect of magma hybridization on Sr and Pb iso- into the upper crust. Alternatively, the composition of tope ratios; symbols are the same as in Fig. 7. Curves illustrate each rhyolite could reflect the ephemeral effect of each mixing between high and low 87Sr/86Sr mafic magma and an average rhyolite composition. The isotopic diversity of the most recent magma recharge episode. In this case, mafic magmas, inherited from lower crust interactions, is compositional differences between one set of coeval largely preserved by the rhyodacites. The comparatively nar- rhyolites and the next could be a record of the response row range of the rhyolite isotope ratios is produced by hybrid- to and size of the mafic incursions, but not necessarily ization and homogenization within an integrated magma system. The fields in (a) show the range of 87Sr/86Sr ratios for of the long-term dynamics and thermo-chemical state igcb, igsp, and the Risco Bayo–Huemul plutons, plotted as of the magma reservoir. Protracted extraction or resi- measured (Nelson et al., 1999; Hildreth et al., 2010). dence in the crust would tend to average out subtle Assimilation of this material would yield higher and more var- ied 87Sr/86Sr ratios in the post-glacial rhyolites than observed, compositional differences; thus, the preservation of favoring a model of self-assimilation within the post-glacial compositional distinctions among the LdM rhyolites magma reservoir. favors rapid melt segregation and only brief storage. Whereas there is scarce evidence for physical inter- also inconsistent with progressive eruption from a action between the erupted rhyolite and intruding mafic zoned magma reservoir. Instead, the compositional dif- to intermediate magma, the extraction of crystal-poor ferences reflect discrete magma bodies that, remark- rhyolite could nevertheless be catalyzed by magma re- ably, produced eruptions over a comparably wide area, charge in the lower reaches of the magma reservoir. similar to those inferred for the Mamaku and Ohakuri Increasing temperatures would raise the porosity of the ignimbrites and rhyolites following the 254 ka caldera- crystal mush and, along with the exsolution of volatiles forming Oruanui eruption in the Taupo Volcanic Zone from the mafic magma, increase the buoyancy of the (Sutton et al., 2000; Vandergoes et al., 2013; Be´ gue´ rhyolitic liquid (e.g. Barker et al., 2016). et al., 2014; Barker et al., 2014, 2015). Rhyolites of dis- tinct composition were erupted c. 20 kyr apart, from Structure and dynamics of the magma reservoir vents separated by only 2 km (e.g. rap and rln). In con- The combination of the basin-wide progression generally trast, coeval rhyolites nearly identical in composition from andesite to rhyolite, the importance of magma hy- erupted more than 10 km apart during both the EPG bridization in rhyolite petrogenesis, and the temporal co- (e.g. rap and rle) and Holocene (e.g. rln and rcd). Rather herence of variable rhyolite compositions suggests the than being the products of small, short-lived, isolated physical configuration of the LdM magma system 106 Journal of Petrology, 2017, Vol. 58, No. 1

(a) (b) (c)

rdne rdcd rdcn (d) (e) (f)

(g) (h) (i) (j)

Fig. 15. Textural evidence of open-system processes in LdM rhyodacites. (a–c) Outcrop photographs of rdne, rdcd, and rdcn show- ing representative examples of chilled mafic inclusions, highlighted by the arrows. (d–j) BSE images of representative rhyodacite plagioclase textures including sieved and mottled cores, resorption surfaces, and oscillatory zoning—all indicative of varied and complex thermal histories. In contrast, rhyolite plagioclase crystals, not shown, are dominantly homogeneous. The scale bar in each image represents 100 lm. illustrated in Fig. 17: it comprises an integrated magma systems. These systems have produced a range of source zone, sustained during at least the last 26 kyr. This eruptive behavior including the sequential eruption of region is spatially extensive and intercepts the ascent of diverse rhyolites, the coeval eruption of spatially and diverse mafic magmas that promote magma hybridiza- compositionally distinct magmas, and the pre-eruption tion, resorption of accessory phases, and the segregation amalgamation of several magma bodies, yielding volu- of crystal-poor melt batches. In the south, this magma minous ignimbrites characterized by isotopically and mingling and mixing is limited to the base of the crystal compositionally diverse phenocrysts (Bindeman et al., mush, resulting in little physical interaction between the 2008; Deering et al., 2008; Charlier & Wilson, 2010; recharge magma and the erupted rhyolite batches. Klemetti et al., 2011; Storm et al., 2011, 2014; Barker Thinning of the system to the north allows for penetration et al., 2014, 2015; Be´ gue´ et al., 2014; Wotzlaw et al., of mafic magma to shallower levels, thereby promoting 2015; Evans et al., 2016; Myers et al., 2016; Rubin et al., the eruption of mingled and hybridized magma. Crystal- 2016). The compositional continuity of the distributed poor rhyolite is periodically extracted and stored only rhyolite eruptions through time observed at LdM and briefly prior to eruption. The composition of these erupted Taupo, post-Oruanui, (Sutton et al., 2000; Barker et al., magma batches reflects the longer-term homogenization 2014) demonstrates the remarkably lateral continuity in the upper crust by magma hybridization, temporal vari- possible in silicic systems and the short timescales over ation in the thermochemical state of the magma reservoir, which compositional distinctions can be produced. and possibly compositional characteristics imparted dur- Neither LdM nor Taupo have erupted high-SiO2 rhyo- ing melt extraction. lite with extreme depletions in Sr and Ba, large negative The repeated generation of compositionally and iso- Eu anomalies, and low temperatures that characterize, topically distinct rhyolite magma batches is an increas- for example, the Glass Mountain rhyolites erupted at ingly recognized feature of long-lived silicic magma Long Valley (Metz & Mahood, 1991; Hildreth & Wilson, Journal of Petrology, 2017, Vol. 58, No. 1 107

8 900 (a) Silicic Lavas (c) rhyolite LdM ignimbrites EPG 875 rhyodacite L. Pleistocene & Holocene 850 6 825

800 Latest Pleistocene Sm [ppm] & Holocene trend 775

4 T [ºC] Two-oxide 750

EPG trend 725 Latest Pleistocene EPG & Holocene 2 700 30 20 10 0 24 Eruption age [ka] (b) LdM ignimbrites (d) 22 23 - 46 ppm Y

20

Y [ppm] 18

16

14 65 70 75 5 km SiO2 [wt. %]

Fig. 16. (a, b) Comparison of Sm and Y concentrations for EPG and Holocene silicic eruptions and central LdM ignimbrites igcb and igsp (Hildreth et al.,2010; Birsic, 2015) indicating that two compositionally distinct post-glacial rhyolite bodies were erupted in central LdM. The enrichment of the Holocene rhyolites in MREE and Y is consistent with the resorption of cryptic titanite and/or amphibole. The destabilization of these phases could be in response to either repeated mafic intrusion or the ephemeral effect of each most recent recharge event. Error bars corresponding to the 2r analytical uncertainty are smaller than the symbol size. (c) Temporal variation in two-oxide temperatures. The Holocene rhyolites are subtly hotter than those erupted in the EPG, whereas rhyodacite temperatures vary little during post-glacial times. Eruption ages were determined by 40Ar/39Ar or 36Cl, or were estimated from stratigraphic relation- ships (Table 1; Fig. 4). Pink and orange symbols are rhyolites and rhyodacites, respectively. Vertical error bars are the range of tem- peratures produced by touching pairs or the minimum and maximum determined by combinations of isolated titanomagnetite and ilmenite crystals, with the tick indicating the average; the uncertainty in the thermometer calibration is 630C(Ghiorso & Evans, 2008). (d) Map showing the distribution of silicic lavas erupted during the EPG and latest Pleistocene to Holocene.

2007). Such compositions are indicative of a eutectic factors such as the local and regional tectonics, the crus- mineral assemblage saturated in two feldspars and tal lithology, the depth of the magma system, and its quartz. The crystallinity of eutectic systems is more sen- volatile contents contribute to the dynamics of rhyolite sitive to temperature than those saturated in plagioclase generation. However, the minerology-dependent re- and quartz, resulting in more variable trace element sponse of the shallow reservoir to magma recharge may compositions in response to large changes in crystallin- also have significant implications for the varied mechan- ity during both cooling and remelting of crystal mushes isms and timescales of the generation of eruptible rhyo- (e.g. Mahood, 1990; Sutton et al.,2000; Bindeman & litic magma bodies and the growth of their source Simakin, 2014). Recharge by hotter magma is implicated regions; this is worthy of further investigation. in the generation of eruptible rhyolite reservoirs in both The similarity between the rhyolite volcanism at sanidine-bearing and sanidine-free systems but possibly LdM and following the Oruanui eruption in Taupo is by different physical mechanisms. The melting of fertile, striking and suggests similar underlying dynamics. sanidine-bearing mush or hydrothermally altered silicic Owing to active rifting and a high flux of mantle- precursors contributed to the caldera-forming magma derived melt, the rhyolite productivity of the TVZ is re- reservoirs in Long Valley (Chamberlain et al.,2014a; markable globally (Wilson et al., 2009). Tectonic exten- Evans et al.,2016), San Juan (Bachmann et al.,2005; sion is often suggested as a catalyst for rhyolite Wotzlaw et al.,2013), and Yellowstone (Bindeman et al., volcanism (e.g. Hughes & Mahood, 2011)andthecon- 2008; Bindeman & Simakin, 2014; Wotzlaw et al.,2015) centration of silicic volcanism behind the frontal arc in systems. In sanidine-free systems, the thermal input of the SVZ, and at LdM in particular, may be related to magma recharge catalyzes the extraction of crystal-poor back-arc extension (Folguera et al., 2012). However, rhyolite and resorption of some minerals, but not remelt- widespread extensional structures are not observed at ing on the scale observed in eutectic systems (Barker LdM and thus the effect of local to regional extension et al.,2014, 2015, 2016; Singer et al.,2016). A number of cannot be confirmed. 108 Journal of Petrology, 2017, Vol. 58, No. 1

xtl-poor 2 km IMPLICATIONS FOR THE CONTINUING UNREST (a) Early Post Glacial The continuing inflation at LdM is interpreted as a re- NW xtl-rich SE 2 km sponse to magma emplacement in the shallow crust Laguna del Maule (Feigl et al., 2014; Le Me´ vel et al., 2016; Miller et al., melting intermediate forerunners mingling 2016). The uplift of the southern lake highstand paleo- to rhyolite flare-up mixing shoreline of >60 m implies repeated similar deform- shallow, eruptible mingled melt crystal-poor rhyolite holding zone ation episodes throughout the Holocene, consistent

crystal-poor melt with the emplacement of a significant volume of extraction magma into the shallow crust (Singer et al., 2015). Zircon crystallization ages suggest that the 600 km3, rhyolitic Bishop Tuff magma body accumulated at a rate of 75km3 ka–1 for 80 kyr prior to its eruption Mafic magma from lower crust (Chamberlain et al., 2014b). At LdM, if the rate of vol- mantle melt + crust melt ume addition modeled to explain the modern uplift is (b) Holocene taken as the growth rate of the silicic magma system Laguna del Maule and the average length of a deformation episode is taken to be 10 years, the integrated volume increase 3 growth of magma mush would be 05km per inflation episode (Le Me´ vel et al., accommodated by surface deformation 2016). The physical significance of magma emplace- rejuvenation of northern source zone ment rates derived from zircon crystallization intervals is a matter of debate. At a minimum, they probably rep-

crystal-poor melt resent an average of many punctuated, high-flux peri- extraction ods rather than protracted steady-state mass addition. To achieve a long-term average flux at LdM of similar magnitude would require 15 magmatic episodes, simi- Mafic magma from lower crust mantle melt + crust melt lar to the one occurring today, every thousand years. Frequent shallow intrusion of magma at LdM, with an (c) Modern configuration ongoing uplift >20 cm/yr Laguna del Maule average recurrence interval of decades to centuries, is continued magma intrusion consistent with the repeated eruption of rhyolite since promotes surface deformation the last glacial maximum and the dramatic deformation of the highstand paleoshoreline during the last 95 kyr. shallow seismicity Wilson & Charlier (2009) suggested that long zircon crystallization histories record inheritance of antecrysts ? ? during the growth of magmatic mush and not the accu- mulation of eruptible magma. Rates of melt extraction crystal-poor melt extraction? leading up to rhyolite eruptions can reach several km3 a–1 as inferred for the Oruanui and a post-caldera erup- tion at Taupo (Allan et al., 2013; Barker et al., 2016). The Mafic magma from lower crust mantle melt + crust melt rate of volume addition inferred from geodesy at LdM is not of this remarkable magnitude, but is similar to the Fig. 17. Conceptual cross-sections of the structure and tem- poral evolution of the LdM magma system. The three panels more modest rates of rhyolite extraction of other do not represent specific moments in time, but rather summar- Holocene Taupo rhyolites (Barker et al., 2016). Thus, ize important facets of the magma system during each eruptive whereas the rate of uplift today at LdM is globally re- episode. The shallow LdM magma system comprises an exten- sive crystal-rich magma source zone that extends beneath markable (Le Me´ vel et al., 2015), the potential rates of most, if not all, of the lake basin. Throughout post-glacial time, mafic magmas ascending from deeper in the crust are inter- magma. The lack of Holocene rhyolite eruptions in northern cepted, providing a source of mass, heat, and volatiles prevent- LdM suggests that the segregation of melt was limited to the ing the system from cooling to the granite eutectic. southern basin. (c) The focus of magma intrusion may have Hybridization and crystallization yield isotopically homoge- migrated during the Holocene as the modern inflation center is neous rhyolite (Fig. 14) that is segregated into eruptible, crys- NW of the most productive Holocene rhyolite center, tal-poor bodies that fed the post-glacial rhyolite eruptions. (a) Barrancas, and the areas of maximum shoreline deformation During the EPG, the abundant eruption of mafic and mafic in- (Singer et al., 2015). The continuing crustal deformation and clusion-bearing rhyodacite lavas in the NW suggests that the shallow seismicity concentrated near the rcb and rln rhyolites mushy rhyolite source zone thins compared with the south- (Feigl et al., 2014; Le Me´ vel et al., 2015; Singer et al., 2015) re- eastern basin where similar products are not observed. The flects magma intrusion and the movement of melt and fluid, highly consistent trace element compositions of rhyolites consistent with the magmatic processes inferred throughout erupted in the north and south suggest, but do not require, that post-glacial times. Consequently, the future segregation of the erupted reservoir was integrated throughout the LdM eruptible, crystal-poor rhyolite appears likely. However, that basin. (b) During the latest Pleistocene to Holocene, less com- such a magma body currently exists, and if so, its extent and mon mafic eruptions suggest that growth of the northern volume, is the subject of a continuing geophysics investigation magma system increased its capability to intercept ascending (Singer et al., 2014). Journal of Petrology, 2017, Vol. 58, No. 1 109 long-term reservoir growth and continuing melt extrac- unrest throughout post-glacial time. Extrapolating the tion are comparable with those inferred beneath pro- volume change estimated for the modern inflation ductive rhyolitic systems that produced yields a rate of mass addition consistent with that which elsewhere. produced the rhyolitic Long Valley caldera-forming Mixing between existing reservoirs and intruding eruption. However, that this unrest is foretelling either a magma has been found to precede silicic eruptions by future caldera-forming event at LdM or an imminent as little as weeks to years (e.g. Druitt et al., 2012; Till eruption of any particular style is not clear. et al., 2015; Singer et al., 2016). The duration of extraor- dinary inflation at LdM has already exceeded these shortest temporal estimates. Volcanic inflation episodes SUPPLEMENTARY DATA usually conclude without eruption, and the most recent Supplementary data for this paper are available at geodetic observations suggest that the rate of uplift at Journal of Petrology online. LdM is beginning to decrease (Le Me´ vel et al., 2015). There is no evidence to suggest that the current unrest ACKNOWLEDGEMENTS is anything but a continuation of the longer-term proc- esses operating at LdM that produced significant de- Wes Hildreth generously contributed samples and has formation of the highstand paleoshoreline during the been a source of insight since the outset of this project. Holocene and frequent eruptions since 26 ka. Future Luis Torres Jara is thanked for invaluable guidance and rhyolite eruptions are likely; however, that an eruption logistical support for navigating the Laguna. Meagan is imminent is not at all clear. Whether these future Ankney, Allison Wende, and John Fournelle are events will continue to be of modest volume or if the thanked for analytical assistance in obtaining the radio- system is building towards a larger eruption remains an genic isotope and electron microprobe data. Amanda open question and is the subject of continuing geophys- Houts is thanked for laboratory assistance with chlorine ical surveys and numerical modeling investigations extraction. Robert Finkel and Susan Zimmerman are 36 (Singer et al., 2014). thanked for careful Cl accelerator mass spectrometry measurements and data reduction at CMAS-LLNL. This work greatly benefited from many fruitful discussions SUMMARY AND CONCLUSIONS with He´le`neLeMe´ vel, Judy Fierstein, Paty Sruoga, Wes The post-glacial concentration of rhyolite volcanism at Hildreth, and the LdM research group. Simon Barker, LdM is fundamentally the product of magmatism Jo¨ rn-Frederik Wotzlaw, and Chad Deering are thanked throughout the thickness of the crust little different for insightful reviews, and Gerhard Wo¨ erner for editor- from that inferred at SVZ frontal arc volcanoes. ial handling. This research is supported by the US NSF Mantle-derived basalt mixes with two lower crustal (EAR-1322595, EAR-1411779 to B.S.S.), the Geological components prior to shallow emplacement. Whereas Society of America (9791-12, 10016-13 to N.L.A.), the the eruptive expression of this magmatism along the Wisconsin Alumni Research Foundation (WARF), and frontal arc is dominantly mafic to intermediate in the University of Wisconsin Department of Geoscience gift SVZ, rear-arc systems such as LdM yield more silicic funds. compositions—possibly catalyzed by regional back-arc extension. Upon ascending to the upper crust this REFERENCES mafic magma mingles and mixes with pre-existing silicic material followed by fractional crystallization Allan, A. S. R., Morgan, D. J., Wilson, C. J. N. & Millet, M.-A. yielding the rhyolitic compositions. The combination (2013). From mush to eruption in centuries: assembly of the super-sized Oruanui magma body. 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